Intermitant wiper problems

Hey all
Here is one for the fantastic experts on this forum
I just bought a 2002 Z24, but also just found a confusing problem
The wipers would be working fine, on intermitant, but after a while, it would crawl half way up the screen and stop. If I turn he wiper off for a short time (about 2-3 minutes) all would be well again for a while, than problem repeats itself.
This problem also happens if on continous wipe, low. (Haven't tried it on high yet).
If I use single wipe, all is fine.
The mechanic at the shop told me he had already changed the switch.
It seems as if motor or relay gets hot and stops system, but is relay involved if on continous wipe? There are no unusual sounds when wiper is working.
Has anyone come across this problem?
Thanks in advance

Hi All
Just an update in case anyone else has this weird problem happen to them.
I checked with the dealership here, and one of the mechanics suggested that since the car has been sitting for a while, the lubrication in the linkages might have dried up, He suggested to spray some lube on the pivot points while system is running so hopefully some would penetrate.
I'll give it a try