Had a similar fault on my S reg Golf. Eventually got worse and wipers stopped working completely. Traced to motor mounting bolts becoming loose. Motor rocked and jammed up the linkage. When on intermittent with bolts just slightly loose the fault was just now and again but ok on full speed. As the bolts loosened the fault became permanent.
Generally you only notice theis problem when driving in the rain - check your motor mounting when it's dry.
Richard Buxton
September 2008
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