[68 TT]

You may have a failing alternator. My dad was having all kinds of electrical problems with his 78 Caddy. No brake lights, turn signal, stereo etc... Turn headlights on and the car would barely run.

Turns out the alternator was barely putting out any amps so it couldn't keep the battery charged. When the battery was fully charged everything worked properly then the longer it ran the worse it got. After a couple hours of drive time the battery would run down and the car would run poorly and eventually it would not run at all.

You could be having a similar problem. HEI's need as much voltage as they can get to run well. A fully charged battery could make it run without a good alternator but as the charge wears down it will run worse.

It would be a good place to start by having the alternator tested to make sure it is fully functional.