Mech. fuel pump failure modes?

Guys, I have a frustrating problem with my fuel system.. and before I start ripping stuff apart and/ or throwing new parts at it, I would be gratefull for some advice...

Here's the deal, I have a megasquirt EFI system fed from a high pressure pump, which is in turn fed from a the mechanical pump, one for pressure, one for suction from the tank.

Untill recently, the return from the pressure regulator was just fed back between the mech and electrical pump.

After installing a big radiator and electric fans, the under bonnet tempperature increased as there was now no mechanical fan running constantly. This caused idel and re-start problems when hot due to the recirculating fuel boiling.

To remediate this, I did a proper return line to the tank, and that problem was solved.

Some days ago, the engine went lean sputtered and stopped. Gauge on regulator showed low pressure.

Lat the car stand for a while and tried starting ut until battery died. Got a buddy and did several attempts at jump starting. pressure increased very slowly. and once we got it to start, drove it straight back home keeping the revs up on the assumption that the electric pump was failing and needed all the help it could get from the mech. pump. Barely made it back, the engine dying as I rolled in to the drive-way.

Changed electric pump, checked everything from the electic pump back through the return line, fine. Pressurte was slow to build, I assumed this was due to the system needing some priming.

today, car started just fine, pressure where it shoudl be, went for a drive today, and as soon as the engine got warmed up , same thing happened.

This time the car died just before I got back home. Walked back to the hous, got a rag and some icecubes and wrapped arround the mech pump.

After the pump was cooled down, the car started after a lot of cranking and drove the last bit. left the engine idling and checked the fuel pressure. A tad below where it should have been, and not 100% stable.

Now, can a meechanical fuel pump fail partially with reduced volume and/ or deterioration of volume when hot, or do they just plain fail?

I assume the system design is OK, as I didn't get this problem straight away after re-designing with the new return line system.

Just to elliminate the mech. pump as a possible issue alltogether, I could allways install an electric suction pump next to the fuel tank.

But this is a bit more involved than just replacing a failed mechanical pump.. if its (partial) failure is actually the case..

So, how have you seen mechanical pumps fail???