2002 Pontiac Sunfire poor acceleration

This car was running just fine. Then all at once one morning, it started stumbling badly under moderate acceleration. It comes back down to idle fine and idles realatively smooth. Under very light acceleration, it does okay. If I try to pull a hill it just starts stumbling & losing power. At a stop I noticed an unburned fuel type odor, like it may be running a bit rich. The check engine light flashed 3 times and then went out & stayed off. It just started this out of the blue. Any ideas? Thanks