There is a small air bleed pasage and hose to the radiator on the top of the thermostat housing. If that is plugged, or the air bleed hole in the thermostat is plugged (check valve sticking? non factory top thermo housing?), the air will be trapped below the thermostat (insulating it from the coolant) and cause the 'spike' on thermostat opening and erratic temperature swings. You have to get all the air out.