One odd thing to look at..... I worked on a car that'd run well for quite a while and then started dieing after a spirited run in warm weather..... Cause was rapidly diagnosed as fuel starvation..... Input to mechanical pump was good, output, not so much..... Obviously needs a new pump..... New pump installed and same thing.... WTF!!!!
Side of the road pump removal reveals the push rod in the extended position and it doesn't change with engine rotation....
Turns out the phenolic part that heat shields the pump and guides the push rod had evidently absorbed oil over it's life time and, with heat, closed the bore enough to grab the rod..... Walked across the street to the local ACE hardware with the block, and "tested" a 5/16 in. drill bit on the bore....... Reinstall the pump and it was still on the car when sold 2 yrs. later when it sold......