yeah, driveshaft problems dont normally show up till you're on the highway. i'm just throwing out ideas.
quenton's suggestion about point jump sounds plausible. i'd never heard of that before.i dont know what causes it. check that the advance plate isn't loose, and everything else in the dist. is in good shape. points and condenser are cheap. put some in and set them with a meter if you have one, otherwise just set'm w/ the feeler guage 'till you can get a meter.
vacuum leaks are easy to find. spray a little carb cleaner around any suspected areas-all hoses, all over and around the intake, and at the base of carb. if there are vacuum leaks, the idle will stumble.
ign. timing for a 68 400 engine is 9*btdc at idle speed with dist advance plugged. point gap is 0.016 with a dwell of 30*. idle speed i have two figures. with and without idle stop solenoid plugged in. i guess thats like a dashpot. i dont know if you have one, but the idle speed is 850 with sol. plugged in, and 650 without. all figures are with engine at operating temp and manual trans in nuetral.
