CEL flat out in 5th
 

got a my00 uk turbo running 350+ bhp
remapped recently etc

thing is during the re-map my headers to up-pipe gasket blew
we were nearly finished anyway . . . .
gasket was replaced saturday

now,however,when i drive flat-out through the gears the CEL comes on in 5th
earlier tonight i went up my favourite bit of road,dark and quiet ;)

got it in 5th,still flat out and when i backed off at the top the CEL came on and the rev counter dropped to zero
i put it in 4th and the revs went back up
like it had stalled then bump started . . . . . .

could this be a bit of sh1te fuel as i did recently fill up with sainsburys 97 RON
(had no choice,believe me) , maybe i should've put a fiver in till i found better but it was only 84p a litre :)
i do run it on optimax and add a bottle every 4th or 5th tank full and never normally let it run below quarter full

i have since added optimax and just put a bottle in now . .

any ideas anyone ?
i'm supposed to be doing a track day this saturday , only a little track , 3rd gear , 85 mph maximum , , but really dont wanna blow it up

cheers in advance
daz