Did you fit the variable vanes in the turbo yourself? The variable vanes need to be set via a flow bench at a turbo repair place, if you bought a recon turbo or got yours recon'd at a turbo place they'd have set the vnt via the little grub screw on the bearing housing of the turbo. If this isn't set then the air flow can be off and and result in the turbo not kicking in at the correct revs that it should, too low down, too high up, etc...