It wouldn't cut out necessarily if it was the headgasket, you would just see the temp gauge hit the roof in about 5 minutes and then maybe burst a hose and see loads of steam if you kept it running. So if it's not over heating then I'd rule out head gasket. Mayo in the oil may be head gasket failure but can also just be moisture. Check your header tank too to see if it's got oil in it.
Could be the dizzy, just get a new rotor arm from GSF, they are only a few quid but as you've said the filler neck is corroded then I'm guessing that you've got rust particles in your fuel lines. You haven't recently run the fuel down to the minimum have you ? That's usually when stuff like this happens as it sucks up all the sh1te in from the bottom of the tank, including all the rust from the filler neck. If it's this you'll need to get the fuel system flushed out somehow.
