are ur diverter valves working effectively?

if not maybe u may have a vacuum leak I had the same issue not so long ago found the leak added t-bolt clamps just to be sure and my 3" Y pipe no longer comes off.

Vacuum leaks equal big problems with these cars.

 

I know how to check for boost leaks, but what is the process for checking for Vacuum leaks?

 

I use the old school method= spray parts cleaner around hoses and listen for idle to raise. I usually on my older cars such as ours replace all vacuum hoses with new silicone and it eliminates a lot of issues. I'm sure someone will give you a newer better way lol.