I'm having a little problem that I'm hoping all you knowledgeable-types out there can help me to solve. Here is what seems to be happening: I have a boost guage in my car that reads in bar. Recently, whenever I pass about the 3500-4000 rpm mark at high throttle, the boost falls off from about .5 or .6 bar to about .10 or .15, whereas it used to hit about .86. If I let off the throttle a bit (to maybe halfway) as I pass that same 3500-4000 rpm, the boost may maintain a level that is around .5-.6 bar. The boost levels at lower RPMs do not seem to have changed. The car has about 38k miles on it, and I bought it used about 3k miles ago. The only aftermarket parts that are there are the blitz blowoff valve and turbo-timer/boost guage.

Has anyone out there had this problem? Any suggestions on things at which I should be looking? Any help would be appreciated.