Help: Frequent car choke

I have a 2000 Mazda Protege (1.8L, 52 K miles) that ran very well until last Month. Two weeks after I had a 800-mile trip, the engine started to cut off. It ran smooth for 30 seconds or 1 minute, then there is a sudden drop of power and the car hesitates. The symptom is more obvious when driving at relative high speed (> 50 miles/hour) but is also frequent at lower speed, especially when I accelerate too fast (>3000 rpm). It is not the Mass Airflow Sensor (MAS, recalled by the company) as it has been replaced in 2002. I sent the car to three mechanics for diagnosis. The first mechanic assured me that it is the engine, not the transmission. The second and third mechanics could not identify the problem, although they replaced crankshaft position sensor. Does anybody experience a similar problem or have any suggestions?