I have a 91 civic dx. The past few months the car will either stall, or
jerk then lose almost all power. The check engine light comes on when
this happens. It happens only occasionally, but during all types of
driving ( highway, 35 mph, or even just staarting up). If i turn the
car off and turn it back on, it resets and the car is fine for awhile.
The other symptoms are the mileage went down from ~35mpg or better
every tank, to 27-28mpg on the last three tanks. The other thing is
when i got an emmisions test my mechanic said it didnt pass the HC at
first (234 reading w/ 220 limit). He let it warm up for half an hour,
then did it again and it passed.

Any suggestions would be helpful. Could it just be the o2 sensor or
something else simple?