On mine, a 2000 Cadillac Deville, it's been because water is getting into the wiper motor pulse board - I think through the electrical connector. Next time, try that. Take the bottom plate of the pulse board off (3 screws), open it up, and look for water droplets inside it. This water causes the board to short out, creating these exact symptoms you are describing. Dry it out with a hair dryer / blower, clean seal grooves, caulk it, put it back together - it should now work just fine. Also, make sure you caulk well all over the electrical connector where it goes into the board and on it's back side, too; I'm convinced this is where the water is coming into the pulse board, on mine. You'll know if you did it right if it does it again, after it rains - this will be your "stress test", which will determine how well you sealed up the cover and plug with calk / silicone. I've been through this four times on my Deville - what a headache it is. Do it right, completely, the first time - including, again, both sides of the electrical pin connector, seal that up good, too.