My friends EVO 9 has a problem

My friend has an EVO 9 and this is what he told me his problem is

ok its an 05 evo has hsc s1 cams ams flash, ams map pipe, ams monster fmic, 3 inch turbo back exhaust and a boost controler, boost holds well at 20 psi but once full boost is hit around 3,00-4,000 rpms car hesitates under load rpm's jump some then come down mostly inhigher gears is when defiantely noticable.

he said after he launched the car racing this happened

I no nothing about EVO's only jeeps if anyone here can help him out that would be great