Buick engine misses

During the first 10 minutes or so on each trip the engine on my Buick LeSabre 2001 misses. My mechanic says he could not replicate the problem. What may be the problem?

Weak spark from old spark plugs? Dirty air filter? Incorrect fuel pressure? Dirty fuel filter? Erroneous engine coolant temperature sensor? Air temperatur sensor? Idle air control valve? Sticking valves?