check engine/ hesitation

Every now and then, when I push my car a bit, the CHECK ENGINE light illuminates, flashing, while the car splutters and has some serious problems juicing up. The light then goes steady, and the hesitation in acceleration continues for about five minutes (so it's a pain driving in the stop-and-go in the city). Then the light stays on and the hesitation goes away, but I sense a decrease in performance (sluggish acceleration). After a few days, the light just goes out on its own for about four days, but then the cycle repeats itself again. I am thinking it's a problem with the primary O2 sensor, which would account for the spluttering (poor fuel-air ratio delivered by faulty readings). The first time it happened, I found a plug wire loose - I thought it might have been the plug wires misfiring, because my mpg went down dramatically - but once secured, the cycle happened again (although my mpg went back up to @27 on my way to work). So part of the problem was solved anyway. Any thoughts/similar experiences? Thanks.