Maybe time to try a bypass hole in the thermostat periphery Neil. Another thought is the efficiency of the water pump given age of car. If the coolant flow was below par it could cause latency in response to changes in engine power demand and speed.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone