new thermostat fitted today. when brought up to temperature at idle seemed to be working perfectly, top hose got hot, then bottom one - in fact all hoses hot and coolant seemed to be circulating, Temp gauge reading 92/94. Then took it out for a test drive and within a few minutes was showing overheating 108 to 116C) and so came back and checked hoses - bottom hose now cold??? Has anyone had these exact symptoms and cured it? All thoughts appreciated.