I need some help guys!

[sam's 72]

I have a 72' with a 1986 305 in my truck right now. It has some what of a miss. It is kind of a wierd thing. It runs great while running down the highway, but when you come to a stop, it has a slight miss, not constanc. Like every 30 to 45 seconds, ( something longer intervils ) it misses. It does not do this all the time, some times it is worse then others. Some times it is real bad when it is cold, other times when cold it is smooth as silk. It acts like it miss fires just every now and then.

This is what I have done so far
New rotor
New cap It has an HEI in it.
New wires
New plugs.
Timing is set 8 degree advance intial, the vacuum advance is working.
Checked all vacuum hose's, none disconnected.
Carb is adjusted with vacuumm gauge, I have it adjusted to where it is pulling max vacuum,
New air cleaner.
When under a load it doe's not miss at all, very very smooth.
I am really stumped on this one. If it did it all the time, I would think that a valve is not set right, or maybe a broken valve spring, but it might go two or three days without doing this, then all of a sudden it's back, when it is doing this, if, I rev the engine, it goes away.
What do you guys think this could be?
Sam