There are three suspects, motor, relay and "handle." I would go to the relay first. It is located under the dash to the left of the steering column held in place with velcro. It is a rather large box. You can disconnect the wires from both ends and connect them directly instead of the relay. What you should have is low and high speed only, no delay's. If that is what you get then your relay is bad.
If you have a DDM you can check the resistance through the intermitant settings. It should vary from I ohm to low ohm, long delay to short delay respectivly. If not then the control may be faulty.
Last is the motor itself. I had one that would not work properly even when changing the relay ( I have two xj's). It was the motor. After finding a replacement, I took the "bad" one appart to see what the problem was. You can remove a cover plate that the wires go through. I found connectors that wipe during operation had little preasure against the contact plate. I simply rebent the spring connector and now have a spare wiper motor.
Should you decide to get a motor, watch out for the connector from the motor to the wire harness. They changed it in 91, it think. The motors are really the same and you may be able to swap connectors. The early ones are round.
One other thing you may want to check up on is the bushings for the "transmission" assembly. They typically get loose, alowing the wipers to have excess play. There are aftermarket bushings available and are easily replaced. Do a search on this site to find the HELP part number.
Good luck.
