Matt, you sound like you're having the exact same problem I had a couple months back (which I still haven't fixed). Drove fine then died. Towed home. Next day, started fine. Drove for a bit, died. Towed home. Has happened 4 times in total.
Here's my thread: http://www.cuda-challenger.com/cc/index.php?topic=87884.0
I initially thought it was an ignition problem, but after getting it towed home, it started up the following morning like there was nothing wrong with it. Got it to a dyno tuning place and they sorted a few small issues one of which was a fuel filter clogged with brown fuel. Thinking it was a bad fuel problem, I dropped the tank only to discover it the tank was spotless, and the lines were clean. Must have just been a bad fuel filter that its internals broke down (it was a solid filter not a clear one, so you couldn't see).
So now I'm back to the electrical/ignition side of things.
There are a couple voltage drops from the fusible link and somewhere from the fuse box, but I've yet to get it towed to an auto-electrician. Only put the tank back in yesterday. I do know my spark plug leads aren't good enough. So I plan on replacing the entire ignition system with an ICE unit, and take it from there.
The cylinder replacement you got, was it the Fuel-Miser brand? They're a steaming pile of crap (mine broke whilst putting it on).
Good luck mate, and keep us informed ... and post pics! 
Hi Russ,
The problems certainly sound similar. Ongoing and really hard to diagnose.
DynoTune did my work for me ('rebuild carbies' which were brand new, removed ballast resister, new coil, leads, plugs, new module, tune). The coil was a Fuel-Miser, I'd say it was fitted because they don't stock the genuine Bosch item any more, even the Repco bloke said people with Falcons have had a lot of trouble with this replacement (no ****, they will overheat!). I bought a genuine Bosch item from somewhere else.
If it were me, I'd have gone for a full MSD 6 digital straight up. But I left it to the experts (first mistake).
Somehow the timing got retarded, it might have moved its way around with all the work on the car. It wasn't done up too tight and the nut is burred, someone had been using a 13mm on a 1/2inch nut. Typical...
I think the fuel side of things is fine in my car, the tank is new but I'll check the filter tomorrow. Although, a **** load of fuel vapour is around and the centre carb is so black. I'm going to play with it tomorrow when the old man comes over, since the car now won't idle it's a two person job. It wouldn't surprise me if the jets are too big and the timing is too retarded (can't tell without a mark). I do wonder why I paid these people thousands of dollars.
Russ I'd give this a go, even if it doesn't work for you it should be done and is easy to do:
http://madelectrical.com/electricaltech/amp-gauges.shtmlSince the awesome auto elec who fitted a new alternator and regulator (and blew all my bulbs that were on and 12V gauges in the process of fitting the wrong alternator I presume) I'll be buying a new clock centre (again), tach (again) and now volt gauge to replace the ammeter - much better charging system as well.
