A80 Decellerates

[TAM]

Hi i'am new to this. but my 94 m reg 80 1.9 TD goes fine on country roads but when I get out on to the moter way going up hill once it reaches about 75 MPH it draw back to 65, but if i lift my foot from the throttle when i feel this happening and put iback down hard it pulls away againe to about same speed again then does the same thing. I have had it to garage on a giognostics and it said there was a problem with low presure @ intake manafold. but there is no oveas problems there even the mechanics didn't understand wat problem could be.