1999 Oldsmobile Cutlass Intermittent issue

My 99 Oldsmobile has recently developed an issue that my mechanic hasn't been able to diagnose. About 2 months ago, I took a short drive thru city traffic and was at a stop light. The light turned green and I tried to accelerate, to no avail. The electrical system seemed ok, and the engine ran alright at idle, but the engine would choke and sputter when trying to accelerate. I was able to pull the car over where I restarted the car after a few minutes. I was able to get it moving again only to have it do the same thing at the next stop light. I had the car towed to my mechanic and it sat for a few days before he was able to get to it. When he got the chance to look at it, he said it seemed to be running fine. I picked the car up from the mechanic and proceeded to drive it for another 2 weeks without any problems. When the car broke down a second time I had been driving 70-75 mph on the highway for over an hour. All at once, the engine seemed to lose power and began sputtering so I pulled over on the side of the highway. Once again, I turned the car off and let it sit for a few minutes. When I restarted the car, it showed signs similar to the previous time it had broken down with one difference. This time the exhaust noise was noticably louder. I tried to drive the car again and it managed to drive alright until I got past 60 mph. When getting over 60 mph, the engine began sputtering again and I would have to pull it over and try again. Once again, I gave up trying to drive the car and had it towed to my mechanic where it sat for a couple days before he got to it. When he got the chance to look at it, he said that he couldn't find anything wrong.

Any ideas you may have would be appreciated. My mechanic is the best i've found, and it suprises me that he can't find the problem. Are there any areas I should ask him to look at?

I appreciate your help.
- Nick