Gen1 SBC runs great when cold.... rough when warm.

Gen 1 350. Stock with a 4bbl and headers. Recently changed the head gaskets.
I can go outside right now in a 40 degree rainy night... with 1 pump of the gas it will fire up in 1 or 2 revolutions of the engine. No cranking involved. Its starts right away. I can let the choke shut off and for about 5 minutes it runs beautiful. Rev it up 5 or 6K and its smooth.
If i let it reach normal operating temp it starts to run like you pulled a plug wire off. It falls all over it's self. If you try to rev it up a little it just sputters as it climbs. It doesnt overheat.
I have the day off tomorrow so i have 1 day to try to figure it out. I figure any "off hand" ideas you guys may have would help my limited brain power. If it is running rough only when hot is the more of a timing issue? Spark?
Thank you very much in advance.

Any of the vacuum controls still hooked up? Is the choke pull off leaking? There are the things I'd begin with. Check every inch of vacuum hose! If it looks old & crappy, replace it!

What is the history? Was it running fine before the head gasket work, and now you're seeing this problem? When you say "stock" does that mean all of the factory emissions tubing is still there? What year vehicle? Did you change anything else when you did the head gaskets? Done any carb work? Just trying to isolate the problem here, so we're not chasing rabbits down old holes.

I would assume you torqued the heads down in the proper sequence and in steps.If not,do a compression test hot.