Screen keeps going blank when I am on Internet,

Hi,well this tab A has sim drawer for a network as well as wifi (which is working well) 3 months old I recently put my giffgaff sim in went in correct way but" sim card not detected "is what I ge,I have tried all things like cleaning soft reset,but no joy,the sim is working perfect in my mobile phone/any advice (I know send it back)just want to try myselfe first also when on wireless and doing important internet stuff the screen shuts off and takes time to appear again thanks x