Flat spot

Been out for a pre race shakedown this afternoon and have discovered what I can only describe as a slight flat spot at 4000rpm. The turbo kicks in and everything goes mad then at 4000rpm there is a second urge to the turbo that gives the impression of a flat spot. Theres no misfire and Nothing has been changed on the car that would have effected the engine. Any ideas????

Sure its not related to the heat today? (if its hot up in Cumbria..)

or maybe a slight boost leak can cause hesitation at certain points, how does the boost gauge react?

Theres a thought......I tend not to look at the guage too much......I will check all the hoses tomorrow morning before the off.