[s-cars] Stumbling and stalling

[EB]

Guys,
 
My car recently started to stumble, as in momentary loss of power.  A few times it happened at the traffic light, resulting in stalling.  What is worse, the stumble also occurs at a high speed, when it is no longer a mild stumble, but a very hard jerk.  Seems to idle OK, though.  Also, sometimes the CHECK ENGINE light comes on and goes away with a light application of throttle.  I also have what has been described lately by a few of us, i.e. sharp drop of RPMs when declutching while coming to a stop.  
Replacing O2 sensor did not help any of the above.
 
Any ideas?
 
Thanks a lot,
Edward