Seems a stretch for running the generator to result in vapor-locking the main engines, but I guess it's remotely possible.
Being a new-to-you boat
, I'd have a look at how the engines are all plumbed to the tanks
, and make sure that there aren't any user-installed 'improvements' that are causing starvation somewhere, or if there is a specific valve/tank/engine configuration that should be followed.
Minute vacuum leaks
can also result in similar symptoms, loose or broken clamps, corroded filter sealing surfaces, old semi-permeable hoses, etc., are all possible candidates there.