vw passat problem

Hello, can anyone help me.
I have a vw passat 1.9tdi, the problem is when driving the car suddenly loses power as if the turbo is cutting out, when this happens theres hardly any pick up when you put your foot down and the car wont go above 80 mph.
This seems to happen at random although Ive noticed that when the weathers hot it happens more frequently its been to the vw garage on several occasions but they cant find anything wrong with it, the last time they replaced the temp sensor, but 2 days later same problem.
Has anyone had this problem or know what it could be?

Thanks Adrian

you didnt say what year it is,or if it has been scanned for codes,but my money would be on the maf.

I concur, have the MAF checked out.

Thanks for the replys, the passats year is 2000. I dont mean to sound stupid whats the MAF your talking about I dont know that much about cars.

its the mass air flow sensor,usually located in the main stream of intake.if the signals from this to the computor go wrong,then poor running results.a scan for codes should tell you if this is the fault.be prepared to pay a hefty price if it is the maf.

O.k thanks Il get that checked out.