[OdyinNH]

Make sure the inside part of the terminals and outside of battery posts are clean. They should shine. If that doesn't help then put your meter on one of the main wires to the fuse box and ground (the meter) for monitoring. It should be close to 12volts or more all the time. I think the Main relay is only for fuel injection. Do the headlights work when this happens?

 

[OdyinNH]

Glad it appears to be fixed. Typically if the lights go out too and then come back it's a dirty terminal.