John Deere 350GLC surging under load

Hey all, this is my first post here. I’m really stuck on this problem and figured maybe one of y’all could help me out. I have a 2007 John Deere 350GLC excavator. This excavator has an EGR and DPF but does not take DEF fluid. Recently the EGR cooler went bad and was dumping coolant right into the intake manifold. It was quite costly to replace the EGR so through a friend I end up getting rid of it. It was professionally tuned and I have no codes. Ever since he tuned it the engine surges under load. It will idle up to 1800 RPMs with no load but once you touch a joystick it begins to surge, roughly 1200-1300 RPMs. I’ve checked everything I could think of. Replaced fuel filters but no luck. I was going to check fuel pressure coming out of lift pump but I figured since there is a sensor there it would throw a code if it had low pressure. The only thing I could think of is an injector failed when water entered the engine but not sure. I’m curious to see what you guys have to say. Thank you, Jack