Fast wipe tries to go backwards

I converted to a mk2 wiper motor some time ago and added the intermittent function with the later relay. All was well until recently. Then the fast wipe stopped the motor turning, but switching back to slow wipe allowed it to carry on. I lived with this until last week when the fuses starting blowing everytime I switched the wipers on. I got a new motor yesterday, fitted it and sat it on the airfilter whilst I checked it. intermittent, slow and fast were fine. I connected it to the wiper mechanism and it started ok. Then it stalled whilst trying a fast wipe. I watched it with the scuttle cover off and the problem occurs when the wipers are at the end of the upwards sweep. The motor seems to have a problem pulling them backdown again. I thought it was maybe an angle problem with the linkages, so I disconnected the arms and motor, moved the linkage round quarter of a turn and put it back together. No change, yet the slow speed is fine. The motor looks like it is fighting over whether to go forwards or backwards. Is there a chance of some sort of current loop passing back through the slow speed circuit? I tried changing the relay, but it made no difference. I don't know where the earth for the wipers is, and the linkages are isolated by the plastic bushes at the spindles.