I had one of these cars and it would not get over 40 mph. Turns out the vacumme modulator was bad and the car was not shifting out as it should.

If you do have a bad modulator, please be advised that replacing one is easy and adjusting it is somewhat of an art. As simple as it is you might want to leave this (if this is indeed the problem) to a shop that knows what it is doing.

Hopefully this is it as this is a very cheap repair.

Pooka