I had a similar problem on my 93 gmc pickup, but it was the delay on the wipers, soometimes it worked sometimes not. What caused it was a faulty circut board on or around the wiper motor I can't remeber exactly. water leaked in thru the seal. I guess it's common. n
Not sure if that would be the problem with your's or not. But when it first started doin it i would hit the circut box and it would work for a while.