I had a similar problem with my F50. The Solenoid valve that releases pressure from the VST wasn't releasing pressure fast enough. The pressure would build up to the point that the needle valve couldn't open and the VST would starve of fuel. Time would be about 8 minutes for the VST to run dry from full. After the motor sat for a minute and the pressuer dropped enough, the needle could open, fill the VST with fuel, and you say, cycle again. It would also cycle if I pumped the bulb. I assume pumping the bulb made enough pressure to force open the needle valve and refill the VST. An easy test is remove the vent tube that runs from the VST to the Solenoid value. It has to be the vent from the VST before the Solenoid. Or, as soon as the motor stops release the schrader valve on top of the VST. If lots of pressure comes out then the VST probably isn't venting. If there's no excess pressure and assuming fuel is going to the VST, then I believe it has to the needle valve sticking.
I also read an article of hot fuel causing excess vapour pressure. I use to lift my motor when I was using the electric. When I restarted the motor, the VST running dry would be more noticable. I believe lifting the motor drained all the water which acts like a heatsink cooling the motor and fuel reducing fuel vapour pressure. I think this was all related to the VST not venting properly.
Between relpacing the Solenoid valve and leaving the motor in the water when I'm using the electric, the problem has gone away.
Mine drove me nuts until I found it. Hope this helps.
If you can access a service manual, there may be a fuel curcuit diagram near the front that shows how the Solenoid valve controls venting. Mine had an excellent diagram that makes it easy to troubleshoot fuel flow issues.
