93 F150 probem

[94XLT]

I have a 93 F150 with a striaght 6 with a prob lol. Its my little brothers truck and when he went to start it he said fired right up and idled great. Went to turn the lights on then boughed down and started acting like it was missing and the engine light came. He also said the volt meter goes up and down? So I had are other brother pull the alt off and have it tested. They said it tested fine? He is only 16 and im 5 hours away so I cant help him to much. At first i was thinking plugs and wires but why would it start fine then bough down and run like crap?? I no its a long shot but has anyone had anything like this happen if so how did u fix it??

Thanks for your time

[big white bufflo]

it could be a engine to body ground look on the back of motor there should be a large braided metal strap going to the body check all cables