vapor lock

In addition to the above, restrictions on the suction side including plugged pickup, collapsing rubber connectors, switching valve (if used) not cycling all the way, restriction in lines generally, WEAK PUMP. AND improper tank caps or plugged vent

A rear mount pump would problably help. I'm a HUGE believe in vapor return systems. If you are in a position to either re-plumb some of the evap. can stuff, or can run a separate return line back, Wix makes two metal can filters, 5/16 and 3/8, that have a 1/4 vapor return port built in.