I just bought a 2004 John Deere 4210 with hydrostatic transmission and a backhoe attachment. It moves ok in low gear at a slow speed, but the engine bogs down (loses rpm) in a higher gear when trying to go at a speed faster than a walk. Does ok on a downhill slope. The backhoe has plenty of power to dig. If pushing into a tree, all 4 wheels have enough power to spin in the dirt. I replaced the hydraulic fluid and filter. Any suggestions on what the problem might be?