The cold run issue is possibly the choke coming off too soon. I've owned a bunch of FEs and they do tend to be a bit cold natured. Does the manifold have the exhaust heat passages? Are the secondaries shutting correctly? And does it "diesel" only after you've driven it and opened the secondaries?
The stock intake manifolds have a passage where the exhaust is forced into a heat passage underneath the carburetor. The heat serves to better vaporize the fuel while the engine is cold, thus quicker warm up and better drivability when cold. I don't know if your aftermarket manifold has this heat riser feature or not. THis is probably not your problem, anyway. Check those secondaries.
This may sound stupid, but are you having float sticking issues? Other thing I'm thinking is that you have a vacuum leak that is showing it's ugly intermittently.
When it's running ugly, can you tell if it is rich or lean? (smells badly of gas or alot of black or blue smoke out the exhaust)
Timing is a key player. FEs are yery picky with timing. Mine is at 8* and doesn't like much more than that and runs like a sloth with much less. Depending on your cam (sayin RV doesn't say anything, there are 100s of different 'RV' cams out there) can give you more leeway on your initial timing as well as the rest of the engine's build. I'd say 8*-10* is a safe starting point.
I will take a stab at this one... I would say it is a fuel issue, since you say it happens when it is warm you probably are suffering from vapor lock. It seems to happen more and more with ethanol based gasolines. Check your fuel lines and make sure the spacer is between the carb and intake. Possibly your fuel pump may also be heading south.