kenwood amp problem

hello, i sold a guy my kenwood X401M amp, when i had it in my 95 neon the thing ran great and powered a pair of MTX 8000  12" subs running at 2 ohms mono.  he put the amp into his 98 sunfire and ran a single pioneer 12" sub (DVC i think) anyway, with his car idleing his amp will cut out, if he rev's the engine it wont. with the car running he gets around 13.5V at the terminals on the amp. he's got a new 105amp rebuilt alt, 60 amp fuse off the battery, all his wires are connected useing terminals and solder. the only concern i have is he used 4 GA welding cable..even though i'm sure that it should be able to handle the load.  he hasn't had his battery tested  but he did say he had prob starting his car the other day and he was able to crank the engine for a while befor the battery even started to show signs that it was losing power....anyone have any idea's? Tri County KustomZ

certified installer

stang351w wrote: he put the amp into his 98 sunfire and ran a single pioneer 12" sub (DVC i think) anyway, with his car idleing his amp will cut out, if he rev's the engine it wont.

This statement tells me he has voltage problems. With the car at idle & his stereo up does the voltage at the amp drop?