This was the conclusion of Bill Fink (Morgan Guru), he believes that the return fuel is heated by the engine with the fuel line being too close and in contact with the engine. In tank fuel pumps can be very sensitive to heat which is why it is advisable to keep fuel tanks with these types of pumps at least half full. Bill's solution is to locate the fuel line away from contact with the engine and sheath it.

Bruce
1964 4/4 Series V Comp (Megan)
1994 +8 (Maurice) tis no more
2013 M3W (Olga)