have a look at my sudden power loss thread. i went thru pretty much every troubleshooting procedure that i could do at home. dealer found nothing and it never had any fault codes.
my first guess is the brake switch.
does your power loss occur immediately from when u start the engine or does it appear randomly?
you said you have a stock tune. have you done any software tweaks of any kind? in the end, my random lag issue was due to a buggy throttle map which i loaded with Uprev
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