Engine Sputters, Idles Rough, and Stalls if Really Hot

[MercuryMarc]

I have put my Holley 1940 maual choke carb back on my car (still rebuilding my HW 5200)- I converted it to auto choke using a spare electric choke spring. It runs great- starts well cold, choke plate and fast idle cam full open after several minutes. But after 20 minutes of driving, the car idles rough, stumbles, and stalls when idling. I reset the idle mixture to get the max RPM and set the idle speed screw a little higher then the recommended 700 rpm idle speed. The problem seems to be worse when it is very hot. I have not had this particular problem before with this carb or my 5200. Any suggestions?
Thanks!

 

[MercuryMarc]

More info-My fan kicks in at 210 degrees and turns off at 195- this is where I have the adjustable temp switch set to. It never gets over 210. But if I am sitting in traffic for a while and the fan is on for long time, the engine idles extremely rough- even as the block temp cylcles from 195 to 210. If I then drive on the freeway, the car runs fine and idles fine when I stop. I think the high temp in the engine compartment is the issue- the fan cools the rad and block but maybe the carb or fuel line gets too hot? Could this be heat soak? Any suggestions are appreciated!!