typically, if it was plugs or the fuel pump it would be worse at full throttle as well.

if its 15 minutes of good running with new plugs it might have too much gas and not enough air is getting sucked through the turbo.

it could also be a preset timing advance/retardation that no longer matches your set up, the variable runner manifold opening/closng too slowly(do you still have that?) the lack of boost.. (turbo lag :P ) since you have a lot of air needing sucked through the turbo instead of the turbo pushing it in..

do you have an o2 sensor gauge? or a scanner that will read your oem o2?