I've had the same problem with my car for a long time. Mine's only when it gets hot though. I would take my car to Indy (60 miles) and it wouldn't miss a beat, get in hot traffic for more than 3 mins and it dies, missing on random cylinders, then won't restart til it cools down, then its fine again. I have replaced so far the: Opti (ACDelco), fuel pump (Walbro), IAT, CTS, MAP, Water pump, Thermostat, and a full tune up with the NAPA Belken wires and TR6s. I believe its the Ignition module (I work at Advance and thats the only module we can't test practically) or the coil breaking down when it gets hot. Other than that I'm thinking tis vapor lock, but I have heat wrapped all of my fuel lines where its even somewhat close to the exhaust, done all of this in the past 5k miles. The girl I bought it off of said it did it to her and I thought the Opti solved it (it was wintertime and it only does it when its hot outside).
A long time ago, I worked on a early 90s cavalier with the same problem (I used to work at a few shops). After looking and looking and looking, I finally Ohmed the injectors, found out one was shorting out when it was hot, causeing the whole bank to stop firing (3.1L batch fire). New injectors and it was on its way. SO....my best guess is coil ($20), ignition module ($80) or injectors. Hope this helps, if anybody finds a cure, please let me know.