Rough idle when hot, fine for most other times

Hi gang.

I'm having trouble with my 225 slant 6 in my 70 Duster. It is a completely stock engine with the Holley 1920 carb on it.

It starts instantly when hot or cold, picks up speed well and idles well when cold while the choke is on.

But when it gets warmed up, the idle surges then almost dies, then isfine for a few seconds. So if you're in gear at a stop light you go between almost stalling and having to hold the brake hard not to move. It does a similar thing when you're just coasting at 40mph or below, so you're not on the gas pedal much and just maintaining your speed. The car kinda surges then dies randomly, not a steady drive at all.

Any ideas what could be the cause? I think I have the fuel mixture right and don't noticeably hear any vacuum leaks,but I could certainly by wrong.

Thanks or any help.

The Big E