[s-cars] @ 16psi ECU is going into Safe Mode. Norm max 21....

[Emmett03 at aol.com]

I've checked all the boost hoses and replaced all the clamps. Car pulls hard and no signs of boost leak until I hit 16, then bam, safe mode(which includes a lot of bucking). Turning the ignition off resets safe mode and allows me to get back up to 16psi. Safe mode limits boost to around 8psi, more bucking.  What could be causing this?

94 S4
Stage 1 chip of unkown origin. 

TIA