If not the turbo, whats causing limp mode??

Audi A3 2.0 TDI BKD.
Turbo is 20 months old (recon) and has done 30k (many fast miles as opposed to town travel), getting limp mode in the usual way - hills 4th/5th 2200rpm. Am i right in thinking its unlikely to be clogged up already? intercooler seals changed, vac lines changed, could it be that vac box on the bulkhead rather than the turbo??? BTW the turbo has been Mr Muscled three times and worked only for a short while as if it had freed up something then back to normal - limp limp hill limp, now limping on straights too with too much power!!!
Any advice greatly appreciated before I replace the turbo!!

Cheers, Ive had overboost and underboost codes which I was told depends on where the vanes are sticking, just with so little miles its hard to imagine its clogged up already!!

How reliable is the vacuum control unit and is it likely to fail and cause limping, as all my vac pipes have been changed and all the seals on the intercooler pipework? Turbo is away getting inspected/serviced, but if it limps on return im at a loss TBH, ideas anyone??