Check your steering linkages... will do this if extremely worn

normally in sitdown rider electric's the orbitrol is what keeps the steer pressure in either direction in check and as you turn the steering wheel the valves in the orbitrol divert pressure to either side of the steering cylinder to tell it which way to move. So i would say its in the orbitrol valve. Since you say you replaced it then maybe the orbitrol valve you put on it is faulty?
- Hooked up correctly?
- Does it have a steering sensor on it at the wheels by some strange chance?(unlikely but toyota has been known to throw in things like this on thier trucks before that you dont normally see on other brands)
- steering priority valve (unlikely also as it only diverts pressure from the hydraulics to provide consistant pressure to the steering above hydraulics)

this really does sound like an orbitrol valve issue to me.

just some thoughts to ponder

Could the pressure in the steer circuit be to high?- just a thought