Electrical

IF THE BATTERY IS good ... Sounds like you need a starter - Its pulling too much aperage from the rest of the electrical system to turn over the engine. Good test to tell if its pulling too much amperage is attempt to start the car with the lights turn on ... they should dim very little when starting - if they dim quite a bit then the starter is drawing too much.

Quite possible that the earth/ground/negative for the battery is bad. This could be degredation in the earth lead from the battery to the chassis and/or chassis to the engine. It could also be corrosion on the battery terminal. If you have a multimeter, set it to DC volts and touch one electrode to the negative post of the battery and the other to the engine block. Have someone try to start the engine while you do this and watch what reading you get on the multimeter. This will tell you how many volts you are losing through a bad connection. I suggest you remove the battery terminals, clean up both them and the battery posts, and refit. If this does not solve you problem i suggest you have the cars earth straps replaced.