knock sensor ???

[VMittal@USCCMAIL.uscc.bms.com]

     I had posted a problem earlier regarding my 1988 Audi 80.  Whenever
     I accelerate over 3000rpm in 1st,2nd or 3rd gear the car immediately
     goes into some mode and I literally lose all power.

     However, if I keep to below 3000rpm until I rteach 5th and then I
     can goto 90+mph easily (over 4500rpm) with no problems.

     I received no suggestions but my mechanic thinks that it might be a
     faulty knock sensor detecting knock and putting the car in some kind
     of safe mode.  For some reason, maybe the knock sensor does not do
     this in 5th gear after the car is around 45mph.

     Any suggestions/ideas/comments? TIA.

     -Vikram.