I had a problem similar to that a couple of times.

Once it turned out to be that the fuel pump push rod bore was damaged, and the push rod would bind up when the block got hot.

Another time, it turned out that the pickup coil in the HEI had lost its locator tit, and was rocking back and forth. This caused the insulation on the wires to crack, and would intermittently short out.

Another time, it was just the fuel filter in the Rochester carburetor.