I have exactly the same problem....The Car is a 73 911T. The wipers work perfectly for a period of about 5 minutes. It runs, stops and parks as it should. After 5 or so minutes of constant use when I switch the wipers to 'OFF' they actually speed up and just continue to run.

I have found that I can switch them off if I time the off switch to co-incide of the next wipe cycle (just when the wipers should park) this however can take a lot of attempts.
Has anybody found what the problem ....and solution is??? I cant believe its a wiring / earth issue as surely it would do it all the time???