I've got an older (2000 era) TB016. When I place a bit of pressure on the hydraulic system during use, the RPM's will drop significanly and will not return to normal until shut down and sitting for a few moments. We just had the hydraulic filters replaced, it's not overheating according to the gauge, visual signs and the resevoir is full of antifreeze. Oil level is full, both engine and hydraulics. I'm at a loss. Need some guidance here.
Most likely, the fuel filter screen assembly inlet port elbow is debris is restricted with debris. Remove entire assembly and clear the blockage from the inlet elbow. If you search "3972" on here, you'll find some threads showing how common that blockage is.