1979 BMW 320 Mystery Power outages

Electrical problem
1979 BMW 320 6 cyl Two Wheel Drive Automatic

I have been having trouble with the electrical system in my 320i. For a while the car would not start, then only sometimes, but would always go with a boost. I had the battery replaced (new) and an alternator put in (also new)2 weeks ago. Driving on the highway in a rainstorm the other day (using lights, and wipers on high), the wipers became slow, the gas tank dropped to empty (was 1/2 full) lights went off, signals were not working, and I was unable to accelerate. When I slowed down and pulled over, systems came back on line.

My mechanic has given up, but I love my little Beamer. Any suggestions on where else the current may be going?