1990 Cutlass Supreme stalls

My 1990 Cutlass Supreme International (3.1l) has recently started stalling out. It seems to happen when accelerating or climbing hills. It doesn't seem to matter much whether the engine is hot or cold - sometimes it dies a hundred yards from the driveway, sometimes after 30 miles. It generally starts right up again. I can often restart it without even pulling off the road.
I checked the diagnostic codes. It reported a 45, and I changed the oxygen sensor (what a pain). No change. Anybody got any ideas?

Thanks.

Check to make sure the egr is not sticking open, also try cleaning the throttle body idle air control and passages.