1971 F250 running rough help

Hello! I have a 1971 Ford F-250 with a 390

Today driving it, it started cutting out in third gear, almost like when you would experience vapor lock is the best I can describe.

Had power in 2nd and starting sputtering out also, to which it finally died. Cranked it over for a while and it started. All of these symptoms were almost mimicking what I have experienced with gas vapor lock on an old Inline 6 f150 I used to own.

Any ideas? Thought I was out of gas and I put some in prior to starting it.

But now, as it idles, it starts to idle very rough, starts out good and smooth and in about a minute, the idle speed drops and is very inconsistent.

Any ideas?