68 Valiant dies and runs lumpy, then suddenly runs fine.

Did you happen to get gas right before it started running like crap ?
Could be you got a little water in your gas. Before you go buying a new carb, etc, etc,
go the the parts store and buy a bottle of Heet, and dump it in your tank, then take it for a good drive and see if it clears up.

Back when I was turning wrenches for a living I saw this happen many times,, car suddenly stalls, runs like crap, wont stay running..etc. Bottle of Heet cleared it right up.

its worth a try.