Have you checked and made sure you have a good clean solid ground connection on the backside of the wiper motor?

That can cause all sorts of issues with slow speeds, no speeds, and so on. There is supposed to be an AU or CU plated contact on the back side of one of the left hand side bolt to the fire wall(looking at the motor from the front)

Solved that problem for good may adding backup ground wire to the engine ground strap. Try adding one of those along with a good shine of both the contact surface and the mating point on the firewall.

Good Luck,

Paul