For whatever reason, it's started to backfire on warmup (right after starting and holding at about 1500 RPM). It's also fluctuating at idle after driving. This started happening out of the blue. I've already swapped the plugs out, reset the timing, checked the valve lash and swapped out the fuel filter. I'm starting to wonder if my coil is on it's way out (I have a whole Crane Fireball ignition awaiting install), but it doesn't seem like that would be the issue here. Any suggestions?

For grins, here's my friend Steve & his wife's Celicas (his is the white one, hers is the red 'vert).

Edit: If anyone knows of one of those grilles shown for sale, let me know.