All four wheels locked up tight when I came to a stop light

For the last two months I've had a problem that just suddenly appears and disappears, and my local mechanic has no clue as to what the problem could be.

Last week as I was driving about 35 mph up a 6% grade hill, suddenly the car started to lose power and sounded like it was having a hard time shifting. I pushed the accelerator to the floor and got the rpms up to about 6000, but the mph as still about 20. When I got to the top of the hill finally, I stepped on the brakes at a stop light and there was a loud pop in the back end and it seemed like all four wheels locked up tight. The car wouldn't move at all! After I pushed the accelerator a couple of times, the car finally limped through the intersection. I pulled to the side, put the car in Park and turned it off. When I restarted the motor, the car seemed fine for the rest of the day.

The next day, while going through a small town with several stop lights, again, all four wheels locked up tight when I came to a stop light and stopped. It wasn't a hard stop, but wasn't an easy one either. As I floored the accelerator, light grey smoke came out the back end for about 15 feet. I pulled to the shoulder, the brake petal seemed to go almost to the floor. I pushed the accelerator and the back end continued to grab and smoke. I turned the car off and it did no good. I finally went off the highway into a dirt driveway and spun around, and it was like the brakes just released themselves. The whole thing lasted about 10 minutes. Since then the car has shifted correctly and there were no problems at all with the brakes.

My mechanic took all the wheels off and checked all the brakes. There were no problems at all, and he said he couldn't see any excessive wear to the shoes, despite the smoke I saw. There was a new transmission put in the car about two years ago. The car has been relatively trouble-free until this bizarre on-again-off-again thing. My mechanic thinks there could be some malfunction in the ABS system, but there are no dash lights that come on when this thing happens.

Last month I was going down the interstate at 70 mph and then suddenly it seemed like a complete loss of power. In about a minute I was going 40 mph, even though I was flooring the accelerator. I coasted to the shoulder, pumped the brakes and turned off the engine, and then everything was fine. This was the first time I noticed anything strange.

The transmission continues to shift fine and doesn't make any grinding sounds. When these strange things happen, it is like the brakes are applying themselves. I can feel the brakes grabbing the car tight. My mechanic asked if I ever used the parking brake, but I never touch it. Sometimes in the mornings when I back the car out, I hear a loud pop sound in the back end when I apply the brakes, but after the initial backup, I usually don't hear that sound again.

Any ideas what could be going on here? I like the car and don't really want to get rid of it, but I can't have a car that is this random at when it decides not to climb hills or go through intersections or go down the interstate.