80% of the time it's fine with zero trouble but about 10% of the time after running it for long hours it will idle too slowly and roughly (as if it's not firing completely) and then other 10% of the time it diesels when you shut it off (usually indicative of too high idle at least on the old Mopars and Chevies I am used to). So what's the deal?

There should be an anti diesel solenoid on your carburettor. EIther that has a bad connection, or it needs to be replaced.