I have a 99 Buick regal that stalls while driving or just sitting still replaced mass air flow sensor an that didn't fix it and no garage can figure out why its doing it can you please help. to describe it better the rpm's stay at about 500 while sitting still an just before it stalls it goes up to 2,000 an the car shuts off, when driving rpm's normally level out at 1500 an instead of going up before stalling out it shoots down to 500 an shuts off. i've never had this issue with any car i have fixed so i cant even pin point the problem so if you know a solution please let me know, i've been told it could be the ignition if thats true then tell me so. Thank you

SOURCE: 000 NISSAN Xterra -- after warming up, loses RPM by 500-1000..... have replaced ignition wires and distributor, checked mass air flow sensor and OK.....

I had a problem on my Nissan Xterra 2000. Symptons sound similar. In my case, it ran fine for a while and then suddenly the engine starts to stumble and lose output. At one point it did not even have enough HP to move. Irregular idling too. Thought it was water in my fuel. Bought water remover from the store and added to gas tank. Problem did not go away. Instead, "Service Engine Soon" light came on. Had to have it towed to the dealer as there was no way the engine could keep up with traffic. Dealer replaced the "Crank Position Sensor" today. Parts $33.00, Labor: $300 (about 3 hrs worth of work). Seems to work fine now but I will wait and see. Dealer also stated that Distributor bearing appears to be noisy and will need to be replaced sooner or later. I will wait for it to go before I fix that.

SOURCE: I`ve got a 99 buick

Have a performance diagnostic performed. And run the LOWEST octane that your car allows. Too high octane lowers performance with the slower burn, essentially less voliatile burn.
Check tire pressures and driving habits.