This sounds like the shaft seals on the turbocharger are starting to go. Under sustained boost conditions, see if your exhaust is blowing a lot of blue smoke or if you smell oil burning. If you are, then you have blown shaft seals. You can either use this as an opportunity to put something newer/better in, or replace/rebuild. If you're going to replace or rebuild the stock unit, you can attempt to rebuild it yourself, or buy a cartridge kit and keep your turbine and compressor housings.