Help with Seeming Unstable Idle

[DreamRyder1963]

My truck has had what seems like a unstable idle since I got it a couple of months ago. The truck almost sounds like it is breathing. It starts at 750 rpm then drop down to 500 rpm and jumps back up to 750 rpm all within a sec or 2. While rpms almost always return, I would like to fix this issue.

Here are the facts on the engine. It is a 350 with a HEI distubitor, Offenhouser 360 degree intake and Holley electric choke carb.

I have done a complete tune up and the only thing I can think of that could be causing is the choke not functioning, since the problem all but goes away once the truck warms up completely. Any other possibilities? Thanks for the help.