Car stalls once engine heats up?

1993 Oldsmobile Cutlass Ciera 149,000 miles. Car sputters on the highway while maintaining a speed above 60 mph. Problem continues until I decelerate, and drives fine again for 5-10 miles and sputters again until I decelerate. I replaced the oxygen sensor, and the car stills reacts the same way. Print out from autozone tells me to check the egr valve and mass air flow sensor. The car nows stalls out once the engine warms up. I'm wondering if the problem is the egr valve or mass air flow sensor, but this is just my assumption. Perhaps you can tell me why it stalls out. Stalling only happens when driving or if the car is in neutral running above 1000 rpms.