Problem: loss of power when heating up - clues for the cause?

[maple_leaf_eh]

Vacuum leak, I suggest. (Not really based on much experience, but I know older engines have pressurization requirements and falling off partway into a drive makes more sense than a whole lot of other thing.)

The other thing would be a clogged fuel filter. My father's old car had that trouble. Let it sit for a while and the particles in the filter fall away from the element and allow fuel to flow.