Mech. fuel pump failure modes?

Iagree, it sounds like vapor lock.. the only doubt I have about that, is that this didn't appear on the first drives I did after I modified to the return line system.. and the drive i had today was substantially shorter, i.e. it seems like something happened during the previous drive when this suddenly happened..

It is certainly not like the occasional vapour locks I had with the "closed loop" regulator return set up...