Golf TDI max speed 75 mph

My Golf TDI estate, 1997, accelerates well upto about 75 mph and then stops accelerating. If I slow down and try to accelerate it is very sluggish. However if I switch the engine off and restart, the acceleration is fine again, upto 75 mph. What is going on?

There are three common things that can cause this; 1) MAF (mass air flow sensor) in the air intake out of calibration, 2) clogged intake & 3) failing turbo. If the turbo is goung bad it is usually accompanied by lots of black smoke, way more than the usual diesel puts out. You can remove the air pipe right before the intake manifold to look in at the EGR area to see if it has alot of carbon builup in there, restricting air flow. You can test the MAF by disconnecting it & see if it runs better. It will not run perfectly & this is not a fix. It will also cause a check engine code to be set.

AN easy way to check if the MAF sensor is bad is to simply unplug it, then go drive it and see if it drives better-if it does drive like new or better it needs a new maf sensor. You're check engine light will come on but it will go out if you disconnect the battery for like 5 min.