Just clean all the dusts and try using the thermal paste... At the tip of the processor...

You should change the power button kit after opening it power button switch will cost you just 10rs you should go for it

Its not old age phone and it will have issue.... .. I think you will be facing ghost touch only.. I will recommend you to chnge your phone if you dont want then you should change your display whether its original or not... And try usingyour phone after a factory reset...

Replace your battery ya phir tm apne phone ke developer settings me jaakr background processes ko limit hata kar no background processes krdo...