SES light on intermittantly again

OK another oddball. My SES light comes on when I am driving over 80mph at a steady speed. It will flash at me til I either hit the gas or drop below 80. It is throwing a po300 indicating a multiple cylinder missfire. I have the MAC headers, no cats, and Hooker catback and the computer has been reprogrammed(generically) to accomodate these.

Any ideas on this one. It is not misfiring as far as I can tell.

TIA

That generic program that you have; what is it? Maybe that program has some parameter blocks missing, which is at that right moment (situation) when you have everything lined up its not computing? Does that problem happen when you go faster, at a steady speed ?
It would better if you had away record a movie of the data at the time of the ses light ie. a scanner.

the SES light flashing indicates a "severe misfire"

Trust me it is not misfiring. Especially not severely. It happens only at steady speed. If I push the gas down some it stops flashing. If I go below 80 it stops. Only at steady speed and it only happens at those speeds. I am going to email the guy that programed the computer, maybe he has some ideas.