I had the same on the Coupe, very heavy rain on the M4 and the car performed a full blue screen of death and re-booted. All the dash lamps and buttons flashed bright then went off. Power went and I decelerated like I had chucked the anchor out. I was lucky it was late at night and I did not have the usual 118d/A3 attached to my boot. A few cars skittled around me as I went for the hard shoulder. All lights went off, dead engine (autobox so no freewheel option) then it came back. It was down to water coming in around the windscreen and going down the back of the dashboard. They re-sealed the windscreen again and it has not done it since.

Everyone loves a Morgan. Even me, unless it's broken again.