Cleve,

As you swapped the hydraulic control valve and the problem went away, the issue is in the valve.

Before you replace the valve, take the top and bottom solenoid banks off the vale [held on with 4 allen bolts] and try to remove the lift spool.

IF the spool does not easly pull out, check for metal in the valve block.

At one time there was a problem with the "velocity fuses" coming apart in the bottom of the cylinders and the ram piston would beat the fuse peices to small particals which would work their way back to the control valve and jam or restrice movement of the lift/lower spool, reducing lift or no lift at all. Also would not produce any codes !!

the only thing i can think of is check the pressure relief valve as the smallest of dirt in it will allow oil to pass ,try blowing out with air line and i would change hydraulic filter.I dont think valve bank is servicable as there are no seals in it.
hope this is of some help