I have an 81 242 and experienced a similar problem. I traced it to a connection from the battery to some blade connections, relays, and other stuff on the driverside fender under the hood.

From the positive battery terminal, there are 2 leads. The smaller one goes to the inside of the fender - that is where the connection was bad. This is about where I imagine the infamous 25 amp fuse would be located, but I don't think I have one. I slid a plastic cover up and found the weak blade connection. I sanded the contacts, used a little dielectric grease, and pinchhed the female end a little tighter - no more problems.

I'm away from my car until tomorrow afternoon, but if you want one I can send you a picture at that time.