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

Are you using 93 octane gas?

Is your exhaust close to the floorpan in any spots?

I think the exhaust might be bumping in some areas but usually only when I get on it. I think I have a broken or weak tranny mount. I always use 93 or higher octane.

Would an oversensitive knock sensor cause a PO300?