Have you checked the coils/coil wires? The wire insulation tends to deteriorate right at the coil which can cause arc/short. The coil wires toward the rear of the engine tend to go first. Also MAF sensor and or knock sensors may be the culprit. Cheers
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92' Claret LSL 282,000 miles 1owner(ME)
07' Pearly WRX 40k Cobb Stage II (sold) 
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