Need help boost problem

[87 boosted]

Ok guys the car was still running at stock boost but here latlely i realized the boost gauge has been acting funny (it is a autometer gauge). What it does is the boost raises it is fine until it gets around 11 pounds then the gauge start fludderin real fast from like 15 to 19 pound and the car just kinda falls on it face. I cheacked for vacuum leaks and cant find any. What could be causing this?

 

[BIGPSI]

could be the cat is clogged up. un bolt the downpipe and drive it,see if that fixes it