Hi. I've been struggling with an overheating problem for a while and I've run out of ideas. These are the symptoms:

The car has always run a bit hot, but lately this has been getting severe. I am constantly having to top up the coolant by several litres a week. The coolant (just water for the moment) boils in the header tank and the pressure is such that the tank has now developed a crack, which adds to the water loss. When the fluid level is correct, the rad is hot all over; when it is low, the rad is luke warm at the top and cold at the bottom. The heater works.

This is what I have tried so far:

Had the engine pressure tested - fine.

Removed the thermostat - no difference, except no heater now.

Flushed the rad - good flow.

Checked the water pump - impeller looks and turns fine.

Flushed through all the hoses I can reach.

Removed thermostat housing - water running through head.

The water flow when the engine is running seems a bit sluggish. I would welcome any suggestions as to what else to try. Cheers