My key doesn't stay in the slot, always reject but car runs fine. What can i do to fix it ?

It sounds like the mechanical part of the switch, or lock cylinder is bad. If you are going to try to replace it yourself, research the correct procedure for your car so that the anti-theft system doesn't kick in.

right on, not really sure if your problem is mechanical or electrical, but bob is right about the batt. it can even cause your computer to act weird. check all connections too, grounds especially