The "problem" with going down the electrical route is that wrong brushes, loose earth wire etc would generally cause problems at any speed, while the OP's problems show after 5 minutes on the motorway (or whatever the exact scenario was).
Everything needs checking, that's certain, but the symptoms aren't consistent with the usual electrical gremlins.
Not going down the electrical route, trying to rule it in or out.
These problems have to be tackled in a methodical mannar.
The actual method is not important but you just have to start at one place then go down the path to the end.
Trying this then that followed by something else just has him running around in circles
I fix engine for a living and have learned the hard way, unless the symptome are well known assume nothing and rule out things one at a time till the culprit is found...
It could be something as simple as the fuel boiling in the float bowl. main jet too big/small air leak , slip ring breaking down , maggy brushes loosing contact with the slip ring ( my problem ) etc etc etc
