I've been having a misfire problem with my GT500 for awhile. Very common reason is an air leak. Previously I went over my engine with a fine toothed comb. Made sure air intake was secure, and verified that all connectors were seated. Disconnected some things I no longer needed and redid connectors on vacuum lines (installed new lines where needed). Did everything I could think of.
I put off buying ignition coils because of the cost. However, problem was still there and I ordered some earlier this week. They came today. Changed plugs and put new ignition coils in.
I don't know why I did it, but rather than loosening the air intake and taking it off as one piece, I completely disassembled it. As I was reassembling and jiggling the air box portion and trying to get it to seat correctly, for some reason I noticed that the air box connector (air filter fits inside and tube to throttle body fits outside) was loose. What? I didn't even know it could come loose. Turns out there were 3 torx head screws holding the connector to the air box. NOW THERE IS A HUGE OPPORTUNITY FOR AN AIR LEAK!
Tightened up the torx head screws, finished assembling and took her for a ride. I could NOT induce a misfire, no matter what I did. I'm not saying no more misfires. I've been down that road - thought it was fixed only to have the problem recur the next day. BUT, I'm glad I found the air leak. Makes me have confidence that maybe this time misfires are gone. But dammit, wish I found that leak before I dropped $240 on ignition coils. Of course without installing the coils I never would have found the air leak.