Sounds like you've checked most of the 'usual suspects,' so I'm gonna throw some long shots out there:
- What shape is your exhaust system in? I think the OEM and most aftermarket brands used 'glass pack' type, but if you have an aftermarket system of the 'turbo' variety you could have a broken-off baffle or other detritus that blocks your exhaust at random times (usually resulting in partial loss of power).
- Though your issue still sounds like fuel delivery to me, have you taken a hard look at your ignition? Run the engine in a dark-but-well-ventilated area--and look for any arcing, esp. around the distributor cap or wires. Usually, problems of this sort manifest as missing at higher speeds, but we're 'shooting in the dark' at this point.
- Check the thin, flexible wire that grounds the distributor base plate; it could be frazzled or otherwise damaged which would cause instant stalling.
I had a similar problem with my dad's '55 Thunderbird. We eventually discovered a fuel delivery issue; though the fuel pump is engine-delivered diaphragm-type it would fail randomly. This makes no sense to me, but my friend who bought the car solved it by bypassing the mechanical pump with an electric one.