Croc1968

I have just changed the thermostat on my Skoda Felicia, 1.3 petrol engine, of 2000 vintage. Since I fitted the whole unit, which includes the temperature sensor, the thermostat and the housing, I presumed that all would be well. As per the manual, when I topped up the coolant, I removed the temperature sensor as well as the cap from the expansion tank and kept the motor running with the heater at full. I then proceeded to add the coolant, noticing that a lot of air bubbles were surfacing. After a good half an hour, these bubbles appeared to cease. I presumed that any air in the system had been expelled and refitted the temperature sensor, leaving the top off the expansion tank through which I topped up coolant to the level marked on the expansion tank between the 2 lines. Throughout this procedure, the temperature gauge did not fluctuate, but rose gently until the fan kicked in, returning the gauge to normal. I then took the car for a drive of around 20 miles, along a variety of roads, which included motorways and town centres. During this trip, which lasted over an hour, the gauge fluctuated to just below the red lines, dropping back to normal. This occurred on 6 separate occasions on both the motorway and normal roads. I noticed that the heating which was quite fierce when the gauge was at normal, suddenly cooled and the gauge would begin to rise to the red marks, dropping back to normal, at which point the heating would return. When I got back home, I inspected the expansion tank to find that the coolant had been expelled through the cap and the amount lost from the trip, amounted to 800 cc's. Any thoughts on the possible cause and solution to this problem, would be very welcome.