i need some advice on coils and misfire codes.

ok so my 2000 GT spd has had the check engine light on since i bought it. i got the car for real cheap and i know it has issues, but im like rebuilding/modding and the car runs good and strong. i scanned the car with the snap on modis and it gave me missfire codes in cylinders 1,2,3,4,5,6,7,8 i cant really feel any missfire while im driving, like i say the car runs great. so its a random missfire. i have replaced the fuel filter, the maf, the spark plugs and cleaned the entire intake, throttle body, iac ect. its all stock by the way. i clear the codes and drive the car and after 30 miles or so the light comes back on. if i scan it right away it will have codes for misfire in only one or two cylinders. i'll scan it the next day and there will be a few more and the next day or so it will be all 8. so im thinking i need to replace all 8 coils? it really isint that expensive. i can get 8 accel coils through Atech for 120 bucks or something. have any of you guys had these misfire problems? you think 8 coils will be the end of it? thanks

it does not have underdrives or anything, the intake is stock other than a k&n filter and the maf i replaced with a new stock one. the spark plugs are new and gapped correctly. i am not looking for an excuse to buy fancy coils i would rather not buy 8 coils i was just looking at the accels because they are inexpensive. i did replace a couple of the coils and if i remember right there was no codes in those cylinders for a couple days then they were back i think. that was a while ago and i dont excatly remember. on monday when i go to work i will scan the car and post the codes because i do not have them written down. does a bad alternator cause misfires like this often?