You deffo have a circulation issue. You may have a blocked heater matix or control valve. I think water pumps either work or don't - you need to start pulling pipes off at various places to chaeck for flow with the engine running - DO NOT DO THIS ON A HOT ENGINE.

You can reverse the flow on the heater matrix and even put a hose pipe on it for flow.

Are you sure you are not getting pressure in the system from potential head gasket failure? This would cause what appears to be boiling. Is there any air bubbles present in the expansion tank when running?