Sounds to me like the resolver Block in the JoyStick. You may be able to get a used one from your dealer. It also could be a drive motor. If your dealer can't help you call Me I'll tell you how to check the drive motors.
Of the 100 or So RC/PT 100 we've sold I don't think we ever had a Drive Pump failure. I've seen a couple of charge pumps and a couple of couplers fail if it was a coupler you'd have no hydraulics at all. Your dealer can easily check the pressures for you but my best guess is still a drive motor, Check the case drain flow on the drive motors 1st. If the steering is erratic it may also be the Resolver Block in the Joy Stick.