Oh no, help me out here 1955 chevy truck.

[yellow400ex05]

Alright so I had a pulsating/surging light issue on my truck, so I was told it was a bad rectifier in the alternator so I put a new alternator in it and a good battery. So tonight I was driving and everything was fine but after an hour or so I noticed my headlights/dash lights getting dimmer and dimmer the longer I drove until they were so dim you couldn't drive it. So I stopped once and the lights went to normal brightness for a second til I hit the gas again and they went really dim. When I parked it and waited for a tow truck I noticed that when I turned on the lights it runs badly like a misfire and wants to die, when the lights are off it runs fine. What the heck causes this!?!?!? Alternator is working okay but of course my battery is dead now.