Hi,
Have you looked at the wastgate, had a problem with my 300tdi no power no smoke no get up and go(standard 110 300TDI), turned out to be the linkage for the wastgate had broken and you could only feel by hand as its under the turbo which ment the manifold had to come off to repair. Three snapped exhaust studs and much swearing later problem fixed and normal power and get up and go back, (and the intake manifold gasket was split in two places).
This was after all the usual items, check timing, air filter, fuel filter, fuel lines, air intake pipes, fuel pick up pipe, lift pump, i went looking at the fuel side as the problem i had was NO SMOKE. Could only get 45-50mph max