2.5STI

Only just recently in the cold my car has been juddering in 5th gear and sometimes 4th....i have been reading posts and i think its overboosting in the cold...

It is a 2.5 conversion and i think it was set to 1.2 bar boost...it has a manual
boost controller under the bonnet...i dnt know if the best thing to do is turn
it down a little could if it mess it up because it was mapped to that boost .

plus i have no clue whitch way to turn it or how much? and dont wana boost it up and cause damage??

ANYhelp

thanks