Air leaks as mentioned above by Greg can give some strange tick-over problems because the vacuum when the slides are closed will likely pull in air if there is a leaky joint. A small leak isn't so likely to show up as a problem when running with the throttles open as there is less restriction so lower vacuum.
Re: heat build-up. I don't have a problem with the tick over long idling periods. I use a new type BT-H magneto but have VM28 Mikuni carburetors fitted so not a direct comparison. I have the carburetors mounted using rubber radiator hose so I'm wondering if that reduces heat being conducted back to the carburetors.
There must be lots of people using standard carburetors that don't have a tick-over problem so likely checking all the little things will improve things.