Golf Tdi 1996 (TURBO CUTTING OUT).

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. :confused:

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

RR.