hiya mate,
Ive had an issue like that, not with wipers but rev counter it would jump to red line and back not altering the revs but just had a mind of its own, i changed ignition amp, spark plugs and leads, cured the problem immensley then changed the earth and saw a massive difference, i dont know for sure but start with the earth, then do the same maybe re-gap the spark plugs?.
Good luck
