My STI 3 has a problem with over boosting.It has been reset twice with the subaru diagnostic equipment but after a month or two it is back.Any ideas?

 

Bad,
Check all your vacuum lines. There should two smaller ones which run from intercooler outlet (this has a brass restrictor in it) into a T-piece, then the other small hose will run from T-piece to actuator.
The other hoses to check are bigger in diameter and should run from T-piece to Boost Solenoid, then from the Boost Solenoid to manifold inlet.
Check all hoses for splits, cracks and abrasions. Failing that then clear out all of your hoses (small hoses including brass restrictor & bigger hoses) and Boost Solenoid with brake cleaner sold at Halfords.
For more info on this do a search on Drivetrain under Boost Solenoid.

Hope this helps...Jacko.