Citroen ZX car runs for a few minutes then cuts out and won't restart for an hour or so

This only seems to be a problem in the mornings. It will start and run for a few minutes, albeit a bit on the lumpy side ( but it has never been good in the cold), then cuts out and will not restart again. Going back to it later in the day, it starts and runs fine.

This started recently after repairing a couple of leaks in the cooling system - thermostat hose connection and inlet manifold connection. I have bled the system a couple of times and am fairly sure there's no air in it. I am still losing coolant somewhere though so suspect the head gasket may have a leak.

I have been told that this could be causing the problem with water getting into the cylinders, but then I would have thought that it wouldn't start in the first place and would happen any time of day.

P.s. When I say cold mornings, I'm in Portugal so it's only about 10 degrees, not anywhere near freezing!