If no oil in coolant as well,tested buy dipping finger
in water at radiator.The oil will stay on finger,then could
be thermostat.You can always take it out anyway to stop
boil up and warped heads.
Take the car to your local garage and have whats called a " sniff test " done which takes around 2 mins to do and should cost you NOTHING.
This will determine if the head gasket has gone or going. My brother had the same thing recently on his Transit connect diesel van for example and it was the oil cooler that was knackered on that. new cooler fitted and his van runs perfectly now so food for thought and process of elimination.