Golf Tdi 1996 (TURBO CUTTING OUT).

[Rogerrabbit!]

Hi all i've recently bought a golf tdi and have had some problems with it....
When I got the car the turbo started cutting out in any gear when it got to a certain number of revs ( AROUND 4000RPM) since then I noticed that the vacuum modulator had a pipe missing that goes to the air box, so fitted the vacuum pipe and it now runs alot better, it doesn't cut out when i'm in gears 1,2,3 but when I floor it in 4th and 5th the turbo cuts out at around 80mph the drop in power is very noticable. Any help much appreciated.

have done the obvious like fuel filter, air filter... and checked intercooler pipes for cracks etc..

RR.