gas gauge freakin out

hi all, well my 85 xj gas gauge is going nuts. its basically goign past the full line but shes not full. if i go around a turn it moves and the low gas light turns on which is ok. all in all it seems to work but just not registering properly. thanks alot, jay

Could be the sending unit. My '98 had a code when mine went wacky. Not sure about the '85.