boost issues

[ZEBS3 at aol.com]

     I have a 91 200q sedan and have been suffering a boost problem for over a year, even though the car spent the majority of the year in the shop with a mechanic that I thought was capable.
     The problem I'm having is a loss of boost around 3500 rpms.  Boost drops from 1.6 to 1.1-1.2 as indicated by the factory gauge.  I'm running the correct plugs and the ecu is stock. I have recently changed the mfts back to a 4-pin from a 3pin,installed a new air intake temp sensor and pressure tested the intake system to insure that I have no leaks.
     The mechanic that had my car did replace one of the knock sensors(engine front),and though he said the computer wasn't storing any codes he was convinced that the problem was knock sensor related.
     Any help would be greatly appreciated.