could have been the voltage dipping because of an electrical short circuit mate, check for water ingress, and for any wiring surrounding the stereo or anything coming straight from the battery, a lot of gooks put the stereo or some other god forsaken halfords trinket straight to the positive with no fuse, and leave the wires when they rip it out again.
Ive had a car fire caused by exactly that.
I bought a manta, bloke disconnected his stereo as he wanted to keep it, and left an unfused live wire behind the dash, I slammed the door at home and went inside.....the wire glowed orange like a cooker ring straight from the battery as I discovered when I looked out and the car was filled with smoke - I risked life and limb to get the bonnet open to disco the battery and thats when I found the wire. Burned myself badly trying to snap it off with my hands in panic, then sliced both sleeves off the wifes best coat trying to break the glowing wire which was now slicing thru pipes across the engine! Eventually used a broom handle and got it off, but the damage was done.
Carefully check all wiring in the stereo area and around the battery, remove any damaged bits that may be stuck together thru over heating, and make them safe using insulated block connector, then see whats working and re-wire slightly to repair it.