fred,

I never heard of a wiper changing it's own basepoint without having a problem with the wiper mechanism.
I think it is something mechanical instead of electrical and that the cause of this problem can be find under the heater's air-intake cover. Remove this cover (just below the windshield)to have a look whether the mechanism is bend or not. You will have to take out the wiper mechanism to reposition it (bend or not).

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)