My samsung tablet 10.1 is freezing sometimes

my samsung tablet 10.1 keeps freezing and also has like a oil slick on screen. when i charge it, it has a x instead of a lighting strike and powers up slow. my guarante has run out and it only occasionaly freezes. what should i do as i dont want it to stop working altogether so how do i know if its the battery screen or motherboard that needs fixing and how do i fix and also can samsung still fix as its a secondhand device and how would i get hold of them, generally there is no problem except freezing every now and then. what should i do? Please help!

Ive done clear cache partition i also tryed to take picture of oil slick mark but it doesnt show up properly so what do i do to get rid of it as i am not aware of it getting wet as i am a secondhand user so i dont lnow wether before it had i know it hasnt since ive had it