According to the wiring diagram, the intermediate wiper speed is controlled by the steering column mounted wiper stalk control switch (B2). The selected setting (position 1, 2, or 3) is sent to the motor (P27). It follows then that either the stalk control switch on the steering column itself is faulty or has poor wiring connections, or the speed control wiring and/or swash plate assy at the motor itself is defective or has poor connections.
I would start by checking the involved connections at both locations (stalk and motor) for both cleanliness and matching wire color for correct location at the connections. There appears to be a pair of two wire connectors on the rhs of the steering column, one above and one below the wiper stalk. There is one multi-connector at the motor. The colors at the motor are, in order, black, gray, light blue, purple, gray, black, pink/black, white/black. The matching colors at the stalk are: P1 = light blue, P2 = gray, black, pink/black, P3 = purple, gray.
You could also use a volt meter at the motor connections to time the triggering pulse intervals received at the motor. These timed pulses just initiate the turning of the motor, and the motor internal electrical swash plate controls the complete rotation (at regular speed) of the motor back to the rest position. Then the motor waits for the next initiating pulse from the stalk control switch. If the timed pulses are as you would expect, then it would appear the problem is with the motor swash plate control assy. I suspect you would have to obtain another motor assy, as these swash plate assys do wear out with time.
For those who love to mess with things, these assys can be taken apart, cleaned, lubed and readjusted, up to a point. I've done it a couple of times.
good luck,