Temperature fluctuations problem
 

Car: Volkswagen Golf Mk6, 1.6 petrol, 75 kW, naturally aspirated.Situation:The timing belt kit was replaced, and after the ?repair? the engine temperature stopped holding steady at 90?C. While driving in any style, it fluctuates?70?75?78?85?90?C. It rises, stays briefly, then drops again in the same pattern.Before the repair, it held perfectly steady at 90?C.The car has 86,000 km mileage. The belt and water pump were original. The thermostat was not touched.Coolant was refilled, but possibly not completely correctly. Currently, the coolant level is normal, there are no bubbles, all hoses heat up, and the heater blows hot air everywhere.I suspected there might be an air pocket inside, so I tried all the bleeding methods I know, but the issue remains.Any ideas?