2002 Mazda MPV Rough Iddleing and running

Engine Performance problem
2002 Mazda MPV 6 cyl Front Wheel Drive Automatic 44K miles

8 years ago after driving my brand new MPV for 300 miles the engine fell appart. It was changed under warranty. I decided to keep the car after getting 7 year free warranty on it and some cash back. The car was running fine for 30K miles, then had rugh iddleing and lost power. The CEL came on, computer read bad O2 sensor. I took it back to dealer, they changed O2 sensor and performed a lot of recalls. This was about 4 years ago, car was running fine until now. Check engine light came on a month ago, computer reads O2 sensor circuit low. There were no symptoms. I deleted the code, it never came back. After a few hundred miles a week ago check engine light comes on again after engine looses power and uneven. Code is cylinder one missfired. I delete the code, it comes back in about 5 miles. Engine starts up easily, runs even when cold. Idleing is very slightly rough after it warms up. When driving does not have much power and engine is uneven. There was about a 10 mile drive when it was acting normal, then again started to act up up again with the CEL blinking. Spark plugs were not changed. Appart from changing the plugs, any ideas? I plan to change the plugs myself, but it looks like I have to take off the intake manifold. Once I do all this work, is there anything else I should be checking for while the engine is taken appart? How can I check if the coils work beside putting from cylinder one to a cylinder in the front for easier access?

Thanks for your help

Gabor