Dave,
It has been at least that long since I have had a KCT. However, there are a couple of things external to the KCT which could give you a stalled error. One is a loss of the azimuth reference signal. I dimly recall a test program which would let you read the ADC data. If so, see if both the azimuth and elevation are indicating correctly. If not, check the signal at the back for the rotator box, and at the DB-25 connector to the KCT. I would think that you would see the rotator run away in one direction or another, however. The other case is if there is a problem with the left/right controls. You should be able to check that at the same points by grounding the appropriate lines manually. Perhaps a solder connection has opened up. In any case, you can check this easily with a VOM and a jumper wire in a few minutes.
If all that is working, then the KCT or software is the culprit. As I recall, there were not many chips there, so it may be possible to fix by substitution. It will, probably, be either an ADC chip or the control driver chips.
I hope this will help a bit. I did the above exercise a couple of times, and both times found a bad connection in my work. With luck, somebody using the unit will have some more specific suggestions, and possibly the specific pinouts and chips.
Alan WA4SCA