1994 Nissan Sentra with 255,000 miles runs great when cool, but when reaches normal running temp, it sputters and dies. what could cause this?

livingdeadgirl; Hard to diagnose with such little info...When the engine is cold, the computer demands a rich air/fuel mixture. As the engine warms, it gradually leans. Could be as simple as a vacuum leak, but without some trouble codes or diagnosing, just a wild guess. Sorry.
Loren
SW WA