truck wont start and when i try to it continues to turn over without the key in the ignition at all....

ive heard that this is not so uncommon with these older models but ive been trying everything..i replaced the starter,cables,battery,solenoid...also new plugs,wires,cap and rotor...it ran great for a couple days after i replaced the starter,battery,etc...then it happened again and almost melted the battery cables...im lost...