John Deere Guru Needed – 2013 John Deere 350GLC Excavator – Loses Engine RPM and Hydraulic Power After 5–8 Minutes (No Codes)

I'm hoping someone has seen this before. We've spent quite a bit of money with two mechanics and still haven't solved it. 2013 John Deere 350G Pre-DEF machine Approx. 4,200 hours Farm-use machine only Maintenance kept current No active fault codes ​ Brief History Issue #1 (@ Approx. 3,200 Hours) Machine would randomly lose travel speed while in rabbit mode and eventually stay in slower mode. A large equipment company (Dobs in central FL) diagnosed: Both main pumps & failed Quoted nearly $189,000 in repairs. We brought the machine home and replaced: Both main pumps & Pilot pump (refurbished) for 15k Problem remained exactly the same. A local mechanic later determined the issue was the dual solenoid valve assembly located to the right of the pumps. Replacing that assembly fixed the travel-speed problem and machine ran faster overall. *Note: Ever since then, the machine seems to work best when two functions are operated simultaneously. With only one function activated, it feels slightly slower. We were told this could be a priority valve issue, but it never affected our ability to work. At that time we also: Flushed the hydraulic system Changed all fluids and filters Performed extensive maintenance on all that could be done which was our initial process of elimination ​ Issue #2 (Approx. 4 Months Ago) Machine began: Losing power under load Shutting down occasionally Displaying fuel restriction/common rail pressure codes Found: Clogged fuel filters Algae contamination in fuel tank Repairs performed: Cleaned fuel tank Treated fuel system Replaced filters Machine then ran perfectly. ​ Current Problem About 4 weeks ago while digging a watering hole, The machine began losing: Engine RPM Hydraulic speed Hydraulic power The symptoms felt very similar to the previous fuel restriction problem, except there are no codes this time. We performed a complete service even though it wasn't fully due: Engine filter, Hydraulic filters (including major hydraulic filter), Full maintenance items, final drives etc etc No change. ​ Current Symptoms Machine starts and runs perfectly cold for the first 5–8 minutes: Engine holds approximately 1,800 RPM Hydraulics are strong and fast No abnormalities Then during operation: RPM drops to approximately 1,300–1,400 RPM Hydraulic speed slows simultenously Hydraulic power decreases at the exact same time Hydraulics make a "woe, woe, woe" sound as its loosing power If I release the controls: Engine immediately returns to full RPM Machine sounds normal again and seems ready to work When I resume work: Sometimes it immediately bogs again Sometimes it works normally for a few more minutes and repeats The problem occurs with all functions: Boom Arm Bucket curl Travel Any hydraulic function I've tried running only one function from a cold start and the problem still occurs same exact way ​ Recent Repairs / Troubleshooting Exhaust System Work A mechanic noted the yellow exhaust regeneration/cleaning light was on. Attempts included: Deleted the machine; Reprogramming/deleting the system Machine then still showed high temp light Returned machine to stock programming, disconnected sensor, further remapping Current status: No exhaust-related warnings, no lights no derate, no codes However Problem remains unchanged ​ Hydraulic Leak Repair Repacked a boom cylinder that was leaking heavily just in case. No change. ​ Hydraulic Pressure Testing Results Pilot Pressure Tested at quick-connect near small hydraulic filter. Results: Cold: 600 PSI Hot: 600 PSI Idle: 600 PSI Full throttle: 600 PSI Deadhead: 600 PSI During power loss: 600 PSI Pilot pressure appears stable at all times. ​ Hydraulic Oil Pressure Ports Checked both test ports. Results: Cold & Hot ~55 PSI & 40 Idle ~85 PSI & 65 full throttle ​ Main Pump Pressure Both pumps produced nearly identical readings. Results: Idle: 0 PSI Working: ~3,000 PSI Deadhead/full load: ~4,500 PSI If hot ground hard jum to 5000 During power loss: Pressure falls to roughly 1,500 PSI Gauge needle vibrates rapidly Under load during power-loss event, pressure rises only to around 2,000–2,500 PSI and still vibrates ​ Solenoid / Control Valve Work Since the previous dual-solenoid assembly had caused a major slow down issue years ago (althought different than this), I replaced it with a known good used unit. Result: No change While removing the lower hose: Hydraulic oil came out Followed by a significant amount of pressurized air for about 60 seconds I traced that circuit to a four-solenoid assembly on the main valve bank. Actions taken: Removed and cleaned solenoids Replaced assembly with another unit Result: No change for both ​ Air in Hydraulic Circuit One thing that concerns me, the same hose continues to discharge air. With machine running: Lots of air came out initially Then oil mixed with bubbles Depending on cylinder position: Boom fully raised = mostly air 75% closed to all the way fluid We allowed it to bleed for approximately 6–9 minutes while cycling functions. Result: Air never completely stopped Symptoms remained unchanged ​ Questions Has anyone seen a 350G lose engine RPM and hydraulic power simultaneously like this? Do all my numbers seem normal? What would cause both main pump pressures to drop and fluctuate at the same time? Is the air we are finding in that hydraulic circuit normal, or is it a clue? Could this be related to: Pump regulators? Horsepower control system? Priority valve issues? Suction restriction/aeration? Electronic derate without fault codes? Any ideas or tests to perform would be greatly appreciated. At this point we've checked a lot of the usual items and are trying to avoid simply throwing more expensive parts at it. This is a farm unit and we are running tight on $$ to throw at it. I recently invested in replacing hydraulic thump with new cylinder and thumb and just bought a rake for it... I need it back to work Thanks in advance. I have pictures and videos i can upload if needed