1996 Buick Century stalling

I have a 1996 Buick century this problem started in October of 07. When I start it the motor run rough and stalls it does this three times and than runs fine. I notice that this only happens when its warm outside all winter long this did not happen. Last fall I took it to the repair shop about this problem and they hooked the car up to the computer and that didn't come up any thing. But now that it is spring and the temp. Outside is warming up the problem is back