audi s3 8l intermittent boost no fault codes

[andy1]

Hi folks my mate has a 2001 audi s3 amk basic apart from a remap and suddenly it developed a boost problem sometimes it boosts fine and times it doesnt, theres no fault codes when scanned. he could be sitting at over 3k revs with no boost then all of a sudden he has boost. Kinda like a shot of noz lol. Any help would be appreciated thanks