My 740 turbo wagon w/anti-lock brakes (ABS seems to be working, light is not on and not burnt out) seems to automatically engage the brakes when it gets hot.

After driving on the highway for 20-30 minutes, or around town with the AC on, the car will not move from a stop with the (automatic) transmission engaged; it just sits there. Apply the throttle and away we go, but with LOTS of drag, the car heats up rapidly, the RPM go way up (to 4500 instead of 2500) and of course my mileage is shot.

After the car cools down, the problem goes away. When the problem occurs, the brake pedal first gets very firm and then gets very soft and travels almost to the floor, and the car loses almost all braking power. There is no brake fluid leak and all fluid levels are normal. I initially thought that it was a stuck parking brake, but the parking brake has not been engaged since the problem first occured last week.

I surmise that the problem is a vacuum problem (I hope) or an ABS problem (I dread). Any one experienced this or have diagnostic suggestions?