91 Jetta loss of power at highway speeds

I have 91 Jetta which loses power at highway speeds after 20 to 30 minutes of driving. It starts to Bogg down and chuggs along at 30 or 40 until I shut it off. Then I start it up again and its fine. What the heck is the problem. Please someone with help on this!!! Vdub changed the coolant temp sensor, but that didn't do much. I'm thinking Cat, mass air flow ... What could it be. I did check in the Mass Air Flow and found oil in it. What would cause this, could this be the problem?

Please Help

VDubber 4 Life-at least most of the time.

Technically it's not a MAF, just an air flow meter. The difference to you is that a little oil won't really affect it & it is alot less common to fail. I tend to go with the catalytic converter, do you hear any whooshing air sounds when it bogs down?

I have the same car and unfortunately the same problem. I replaced my Fuel Pressure Reg. and the car picked up an additional 10mph. I spoke with one of the master techs at my shop and he suggested to check the cat. conv., but mine is gutted out. I know for sure it is fuel delivery restrictions. Changing your filter might help, but if you feel the second fuel pump which is next to the filter you and you feel heat, that is the main cause. I also found that my accelerator cable is broken so you might want to check that.

This may be way off but then again, maybe not.
A number of years ago, I had a similar problem with a Nissan. It consistently got worse -- occurring more frequently and speeds dropping even lower. Turned out that one of the cylinders was not working for whatever reason -- basically the engine could not create enough power to sustain those speeds. It finally died trying to climb a mild hill.