ISOs are cool cars....dont see many in the US anymore

If you have an electric pump, it should be mounted as close
to the petrol (see, we can talk Engrish over here) so that the
pump is pushing fuel. Electric pumps are poor at pulling fuel.

How far from the tank outlet to the pump? Since the electric pump
is pushing, the fuel should not vapor lock. However, the line
from the tank to the pump could vapor lock....the pump doesnt
have enough suck to pull out of the condition until all cool down.

Another idea is the pump getting weak OR you are getting a
low voltage condition to the pump that is caused by heat.

mike in tucson