Vauxhall Astra astra ac off due to high engine heat

Hi I need help! This car is never out of the garage! Latest problem has cost me over £300 so far and still not fixed. Warning comes on the dash ac off due to high engine heat. Temp gauge drops to 0, fan kicks in. Rocker cover gasket has been replaced, sensor on the left side of the engine has been replaced. And the right has been replaced. Engine isn't over heating. The air blowers was not heating up as hot as they normally did, it seemed to rattle when accelerating, not sure if it has anything to do with it but all started at the same time. It started after the rocker cover gasket was replaced! Coolest low light is on but it has plenty of coolant to be fair it's been like that for a year plus, my garage can't find what's wrong with it, any help would be amazing. Sorry single woman who doesn't have a clue