It could just be that the idle rev's are too low.... In my old cavalier the amp used to go off when I was in stop/go traffic. When it went in for a service I asked the garage to check the electrics and they found that the idle was set too low. Also when I used to turn the blower or lights on it would sometimes do the same.

Rgds

Bill

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MK II, (Blue but should be Amber) SN: 118
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MK II, (Blue but should be Amber) SN: 118
MKIIa