E30 endurance racecar misfires

[Palmettogasman]

We have a 1987 325i that we race in a stock endurance series. It ran well at Daytona but the next race at Sebring it developed a massive backfire during the pace lap at the upper rpm range under load. This had occurred at other races as well but we changed fuel tank/maf/fuel pumps/ecu/coil and thought it was fixed. We changed plugs at Sebring and it ran well for 5 hours until a long shut off during a red flag then it began to misfire again.
Any ideas where our gremlin may be? The lift pump from the saddle tank is the only bad part we currently found but that wouldn't explain the misfire with a full tank.
Thanks