what do you mean by lose power? Does the engine shut off? Does it lose some of it's power?
I'm guessing that it loses power all together. Check all wires attaching to the alternator, check to see that your starter is bolted in tight and has a good connection to the battery. check the little wires on the solenoid,( they are very small inside the insulation, mine separated completely, and was the source of many frustrations).
Other than checking all of your connections, I don't have any great advice. Electrical problems can be a headache, but there's a lot of knowledge and experience on this site. Members helped me get my EB going last weekend after sitting for over a year. I had all kinds of issues, from bad gas to shorts in the wiring.
Hope you get it figured out