Not sure of the component location in a 91, but you have three items that could cause this, the wiper switch, the interval governor, and the wiper motor.
First guess is the wiper switch, or multifunction switch on the column, since you say bumping the column fixes it. Change it, it's not that expensive and it's relatively easy to do. Not easy, just relatively easy.
The interval governor is the box that operates the wipers in the delay mode. It may fail and cause some of this, but I have not had one do it yet. Not sure of the location in a 91 car.
The motor is also a long shot, but it could happen.
If the plastic wire connectors are still in decent shape, I would change the switch first.