Curious front wiper and engine juddering problem

Hi,

'58 petrol VRS, rain sensing wipers, 102k miles.

Last week in the rain on the way to work the car started intermittently juddering though didn't seem to have any hesitation or loss of overall power. Acted the same on the way home until it hit a sudden downpour, juddered to a halt with the idle revs surging between 800 and 2000rpm before I pulled off and called for roadside assistance - suspecting coil-pack issues. The rain had stopped by the time help arrived, and after checking things (the only fault code was a 'generic' engine management one) he followed me home with no noticeable recurrence of the problem. As I pulled onto the drive, I gave the wipers a sweep and the engine surged again. We repeated this a few times and each wiper sweep caused the surge until after a few wipes it stalled completely.

Obviously the wipers turning on full-speed in the downpour had been what caused the earlier problem and the wipers being on auto meant I hadn't been linking the wipes during light drizzle with the juddering earlier. Rear wiper, A/C, headlights etc had no effect on the engine - so it had to be the front wiper motor shorting.

Checked fuses and all were OK, and wipers themselves still seemed to work smoothly.

Finally got the wiper arms off today for a look under the scuttle panel and everything looks ok - not full of water, nice dry clean wiper motor connector contacts, only a little bit of 8years worth of road dirt and leaf mulch, and annoyingly no stalling or surging when the wipers are turned on! It's no longer raining and everything under the bonnet's had a couple of days to drain/dry out so I'm suspecting a loose connection or damaged wiring that only showed up when it was thoroughly wet.

Any thoughts on where to look?