The FPR was designed to work from a vacuum up to ambient air pressure. Asking it to work under boost is probably out of the design spec.

I supercharged a (non M-B) car I owned a while back and had to replace the FPR with an adjustable one that was designed to work under both vacuum and boost.

Just my $.02.

__________________
"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles