or you have a boost leak, before i got my intercooler setup, i had a small leak in the orange gasket thingy, that did the same, 1st and 2nd wasnt really noticable as you go through them to quick, but 3rd and 4th was very noticable as thats when you stay in it for a longer time.
i would check yer pipes and anything else between the turbo and the inlet.
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