Pulling my hair out

The fuel tank is a 4 gallon cell under the hood that gravity feeds to a belt drive gear pump.
The Optima Yellow battery is in a box in the trunk held down with two 3/8 studs so it’s solid. Goes to a 4 post master switch that the alternator is also wired to.
The tranny was built in 2017, Coan 8” spragless converter, Coan pro-brake valve body. I change the fluid at the beginning of each season, and haven’t seen anything strange in the fluid. I considered the trans brake trying to reactivate until it also happened off the foot brake.
Plugs are gapped at .030 as they have been for the past 4 seasons I’ve been using the injection system with methanol. Other than freshening up the motor nearly everything is the same as it’s been since 2017. I was having some issue last year with the shift box. As soon as I’d launch it would shift to second gear. Totally unhooked the box thinking maybe it was shorting out, but it didn’t change the sputtering problem.
My next thought was maybe it’s something in the barrel valve on the injection. However I don’t understand why it would happen only on the track, and only at the starting line. Throttle response and fuel mixture is fine the rest of the time. I use a wideband gauge to monitor the air/fuel.