problem at high rpms

[zrk666]

i was street racing my friends Z28 and noticed that the turbo was popping at around 4k rpms but has never happened before i put the manual boost controler on. the boost controller is set for 11-13 psi on a stock turbo. the air fuel guage is still reading rich. the car would pop like that about every 1k rpms.

 

[zrk666]

hey is any one going to give me a guess or an answer or am i going to have to find out the hard way and take it to a garage