Audi 80 TDI 1.9

Curious problem on 1994 Audi 1.9 Tdi: (sorry for my english...)
driving at 150 Km/h, engine loss power and goes down to 120 Km/h... If I rise my foot and press gas again, power comes back and speed goes to
150-160... but some minutes later the problem appears again... I repeat rising my foot and pressing again and engine recovers power for some minutes... Driving at 130/140 Km/h the problem does not appear... What is the problem? Filters, pump, everything has been checked but problem is still there...
thanks for help.
Tj / Spain
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