I'm not sure you're getting the full advantage an isolator has to offer when you tie the output side ground to the input side.
Ideally, you should have a supply for the output side (26Amps) that is isolated from the input side (5V side) via the isolators. Indirectly, it also means, you should ideally isolate the 2 supplies. Otherwise, the effects of the ground currents running into the low voltage side and disturbing the PIC will still continue to haunt you.
If it is the supply noise due to RPMs increasing, a simple half wave rectifier at the input of your 5V regulator should keep the circuit happy. I've seen this kind of problems when the alternator starts spiking the battery voltage at high RPMs.
Cheers
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