My wife’s Peugeot 207 keeps losing coolant intermittently. There is no trace of a leak, no water from the exhaust and no 'mayonnaise' on the oil filler cap and the oil level is stable. The only thing that gives a clue is the temp gauge fluctuates around the 70 deg mark where it used to be stable at 90 deg. Had the system pressure tested and it was ok, any ideas?