Losing boost st 4500 rpms

[Alilio, Allan]

Is there anything that cuts back at 4500 rpms.  I can hit 1.7 bar then when i get to around 4500rpms the boost will slowly decrease.  I changed bypass valve to hyperboost valve and it helped a little.  Checked Wastegate, Wastegate Frequency valve, knock sensors, mass air flow all hoses and boots and still boost drops on me.  Please someone help.  I have taken it to 2 Audi shops and they cannot figure it out as well.  Compression test was done and 170 across the board, turbo was also checked and it is good.  Mods on car are K&N filter, hyperboost valve, straight through exhaust no cats, Hoppen chip.

Thanks,

Allan