I have a 95 GTP that will randomly run roughly especially at idle. When this happens all of the lights will dim and the gauges jump. At highway speeds it runs ok until I turn the headlights on then the gauges jump again and everything dims. I had a mechanic work on the car and he replaced the water pump, power steering, crankshaft sensor, and alternator so I know it's none of these. WHAT COULD IT BE?