Pontiac Repair: 2000 Bonneville shutting off and more

Question
My 2000 Pontiac Bonneville (3800 V6, automatic trans.) has my long time mechanic stumped. I’ll run through the scenario and if you could, please give your professional views on this.
While driving on the highway the rpm’s jumped and dropped several times when accelerating. I made it to work though. After work, I decided to drive around a bit before getting back on the highway and at a traffic light; it revved up and down a bit then shut off as if I turned the key. Started right back up and got me about 100ft then did it again, one more time and into a parking lot where I had it towed to the shop. My mechanic check it and he thought that changing the fuel filter would fix it. He did that as well as cleaning the carburetor and the jets. I picked it up and drove for 3 days and all was fine.
Went to fill up and was notified by the guy behind me that gas was leaking out under my car. I decided to try and drive it home but at every light, it revved up to the point that the car wanted to move forward and then shut off. After several restarts I pulled into a lot and towed it again. Mechanic looked it over and no gas was leaking for him. He then changed the MAF sensor and the IAC valve. Said he ran it for an hour and drove and all was fine.
I picked it up, drove a block and it started a lesser revving, got another block, happened again, one more block and it shut off.
Back to the shop.
He examined the car for days and found an ERG valve that was moving too much so we replaced that. As soon as he did, the car died again (for him) but has seemed OK to him since. He still has it so that he can put some extra thought into this before returning it to me.
What could be the problem?

Thanks so much for you time and expertise.

Answer
wow a lot to mull over here. with out seeing it It's just a guess. but I have a few thoughts. that engine in that year was plagued with phantom stall issues. (Sorry) but a few things that could be checked. a leaking fuel pressure regulator meaning fuel leaking into the vacuum line when the car is running. your cenario would fit that criteria but it would happen pretty constant and not every few days. another thought would be a crank shaft position sensor when they go bad you will see a spike in the RPM's (possibly) then a stall just like someone turned the key off. again what you said happens. what needs to happen is someone drive this to get it to act up with a scan tool on the car to see what is missing when it stalls out. the problem with these is trying to get them to act up.  I'll assume it never turns on a service engine soon light other then when it stalls it doesn't stay on? if so that would be a thumb print as to some sort of a fault the engine computer sees and then store a trouble code in history. good luck :-)