Any leak in the intake piping after the turbo will cause you to run rich. This is what wankel was also saying. But another very very common problem is spark plugs and wires. Use only stock or NGK wires, and sometimes Magnacores work. Almost everything is is a big risk. Trust me
For plugs on the turbo cars run only NGK copper BPR6 or 7 ES plugs. Platinum is no good with boost, but I havent tried the Iridium plugs yet. The regular NGK coppers are good though because they are cheap (<2 bucks) and on a turbo car you should be replacing the plugs every 10k miles at the latest. Also, its VERY important to have them gapped correctly. For up to 20-23 psi, use a gap of .028. 25 psi and up, you'll likely need to go a little smaller. I run .026, or the car wont run
Hope that helps.