Blinking Check Engine light and hesitation, shaking.

1999 Nissan Maxima, 112,000 miles, 6 cylinder. I have read many of the questions, however, I am not sure if mine is different. About three weeks ago, I went to have my oil changed, they told me that I have an oil leak at my O sensor. Went to my trusted auto shop, they replaced O sensor last week. This past Saturday 2-24-07, the car seemed to lose power upon acceleration, shook a little, then returned to normal. A few mintes later, the same thing. Always from a stopped position, always between 20 to 60 mph. The check engine light blinked about 5-6 times, then stayed on. Took it back to shop, they checked the code and said there was a misfire at plug # 1, so they switched it with plug # 5. Took the car for a drive, check engine light came on again, it threw some type of transmission code. I had the transmission serviced, they found no other transmission problems. I picked up my car Tuesday 2/27, and that same night more hesitating, stuttering, etc. The check engine light blinked 5-6 times then turned off again. It is now Thursday 3/1/07 and still hesitating and stuttering occasionally. Check engine light still off. Could this be related to fuel pump, O sensor, plugs. Not sure what to pay for next. Thanks in advance. : Also, another thought. Did not start the stuttering and shaking until after the O sensor was replaced. Coincidence?