try looking at the battery terminal clamps. sometimes this can be a symptom of a bad earth or bad overall circuit. Had a terminal fracture on one of my old cars which did as you described. Replaced the terminals and everything was right as rain.
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Gone--- 02' ABP ITR 6MT | 06' BSM 130i 6MT - RIP | 99' Altezza RS200Z 6MT | 02' WRX STI 6MT
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