On my son's 2m6, it starts fine and has no miss. However when we drive it for about 5 or 6 miles at highway speeds, it will act like it loses power and then backfires loudly. It can be driven slowly home with no backfire after that. The engine is not hot at all...

After sitting for a little bit, it can be driven again at highway speeds until the backfire starts.

I am wondering if its an ignition coil overheating or something else. Has anyone had this happen before?

Although I've never had the problem before I've read in many posts that the tach filter might be causing the problem. Try unplugging it. If everything works fine then you can get a replacement from Rodney Dickman.

The tach filter is a small silver can with a single wire (I believe) that is located just under the coil and is plugged into the distributor.

Good luck,

Carroll

Or the infamous ignition module

My Fiero was doing that just before the coil went out!

Make sure its a Delco Coil!

That has happened twice to me now. The first time it was the ignition module, the second time the ignition coil. Since you can drive it slowly, I would say it's the coil.

oh so THAT'S the tach filter. i thought it was just a big capacitor. My '85 GT has one of those, but my '87 GT has two of them (dont ask me why, i just got it like that!

maybe i should replace mine just for the heck of it

I replaced the ignition coil and it looks like that was the problem. It was failing (not all together) when it heated up. Thanks to all for the assistance. This forum is a real value to all that use it.