I had a similar problem a few years back. Car would run a few good laps then fall way off( 1/2 sec) starting in top of 2nd gear and to the stripe. Then it might not act up again the rest of the night, or might do it 2 or3 times. Tried ice, moving stuff around, finally after two months of guessing I got lucky and swapped out the 6al box. That fixed it. Mine fell off a lot more, but otherwise similar symptoms. With having a spare box, it's a quick possibility to rule out.