Sorry, forgot to mention that the fuel line to the carb hasn't changed and it is ending well before the fuel strainer. So that can't be the problem either.
The float valve looks similar to this one:
https://www.macsautoparts.com/model-...8-24872-1.html
With the exception that it hasn't got the two cutouts on the float-side of the valve. Also not cross-drilled.
I'll switch the condenser for a spare new one I have here, maybe that solves it although I doubt it. The problem seems too random for that to be the cause. After all why should it return to perfect working order after sitting just half a minute, it won't cool off much inside the distributor. But maybe it really overheats because of low resistance...
Timing has been checked last week, but I'll double check when switching out the condenser.
Thanks for now, I'll report back.