Turbo doesnt really get a chance to spool up fully in first or second before you hit redline?! So i would say the problem is not gear dependant mate, and the speed you a travelling again should not have anything to do with how your boosting (50mph in 2nd is totally different RPM's to 50mph in 5th for example).

As above, it does sound like a fuel/ignition problem ,either that or your so low on compression because of ring wear it just cant go any faster!! lol!

Again as above, have you changed something or is this out of the blue?
What boost are you running and does it hold that steady?
Whats the spec of your car?
Have you ever had it setup?