I had a signigicant power loss with my 78 last year, it was intermittant. After treating the fuel, running diesel purge, and checking the timing, it noticed the fuel line there was moist. Ran my hand over it and it smelled of fuel.
Took it off and looked ok, but could blow through one end with the other plugged and it would leak air easily.
Just replaced and the problem went away quickly. I am guessing that it was intermittant with low fuel in the tank, and with different temps. With a full tank fuel will usually pour out the line, not so when running on empty.