I just bought a 2005 John Deere 6420 and it has really stiff steering, and really slow hitch movement. My first attempt to resolve this issue was to drain the hydraulic/trans fluid, check and clean the sump screen, and replace they hydraulic and transmission filters. I then refilled with new JD HyGard fluid, but this had no effect on the problem. Raising to max RPM has very little effect - maybe just a very slight improvement. What would you do next?