Vauxhall Corsa overheating

Ok, the car doesn't overheat when it's driving. When am idle it'll creep to 100 but not in the red zone the fan will kick it and send it back down to half and it'll continously do this. When am driving it sticks to half so it's when am not driving and engine is left running. Thermostat has been replaced headgasket test done, radiator switch and temp sensor replaced too. What can it be then? I've doen atleast 7k miles with this problem but could never fix it. Suggestions I've had is waterpump timing belt. I don't believe it to be the case, as water is circulating fine the whole process works fine. Uncle suggested it's the radiator it self which is very rare that has a airlock or has contamination and would need a flush. Took it to radiator services they said it's best if I took radiator off my self first bring it down and they'll do me a flush but it's rare to have contamination in the radiator. He did say however that because the first owners were old couples who only did local driving and probally never exceeded 30mph it could be the case, and that's right as when purchasing the old couple had car garaged as second car looked very new low miles and he did say i've not drove more than 40 lol. Now am leaving it to the experts, any questions? Heaters and everything working fine etc.