Things I'd check:
1. Water pump - make sure it's actually working. Remove a hose to see what the flow is (heater hose is good). Take care - the water will be very hot and only do it for a second or two but shutting engine off.
2. Air lock - you may have an airlock in the system that needs bleeding out.
3. Compression test - may show any headgasket problems.
As the temperature went up after it was topped up, I'd suspect an airlock as well.
