My car does this alot too. Its only really bad when i'm idling, I can see the battery gauge bounce around and the lights dim. It used to be much worse but I installed a 1 farad capacator which seemed to help. I also replaced the battery a month or so ago and its still doing it, so I have a feeling that the Probe's alternator just cannot dish out enough power for a large system. After talking with some Audiophiles we came to the conclusion that the best choice would be to get an Optima Gel batter and a stronger alternator. Also I would NOT reccomend an underdrive pully. Good luck!
"New Dan" Isenberg
'95 Probe GT
Crappy Paint, Dual Exhaust, 100x4rms Alpine amp pushing Infinity Reference mids and highs, 450wRMS MTX amp pushing 3 MTX 6000 10" Subs. 17x8 Momo Sports and Pheno's waiting to be put on the car.