My 2002 Wrx will not boost above 15psi. I had no problems with this until recently. In 1st gear when I hit 15psi the boost won't go any higher. On all other gears when I hit 15psi the boost again won't go any higher but the engine shakes a bit. It feels just like boost cut but I do not get any CELs. If I ease off the gas or shift to a higher gear everything is normal again. Other than that, the car drives and idles perfectly.
This started happening about a month ago. Since then I have had the head gasket replaced because it was leaking. I had a small exhaust leak fixed on the y-pipe. I cleaned the Maf sensor. I reset the ecu. I have emptied and filled the gas tank 3 times with 91 octane fuel. I have added octane booster and fuel system cleaner to the tank. I checked the fuel filter and it's clean. There doesn't appear to be any vacuum leaks.
The car has an IHI VF30 turbo, STi 565cc fuel injectors, Walbro 255lpr fuel pump, turboback exhaust, APS cold air intake, and a stock TMIC. The ECU has been re-flashed with Cobb's VF30 91oct map.
The ecu map sets the target boost level at 16.5 +/- .7psi. Could something be bypassing the ecu's levels? I checked the boost controller and it's two-port and it is electric so I assume it's going back to the ecu for control. The wastegate actuater can't be adjusted since it's an IHI turbo. The restrictor hose came with the VF-30 turbo but should I get a restrictor pill with a larger diameter hole? Should I just remove the pill? This problem is very specific since it only happens when I reach 15psi. Anyone have any ideas?