Turbo problem
 

I have a 2010 Cobalt ss with gm stage 1 upgrade and when I got it back it was boosting about 150kpa(21.7psi) and a month later some times hits 105kpa(15psi) almost like went back to stock, but it it goes back and fourth, almost like it's not holding boost. I looked over the lines didn't notice anything anyone with any suggestions?

Do an actual boost leak test not just a visual inspection

Would the gm dealer be able to do it ? Live in a small town it's about the only place around and there not the best here