Sometimes car wont start

What's the symptom when it doesn't start? Measure the battery voltage with the engine off and again with it running, and holler back with those numbers.

Still don't know what the symptom is. If the engine cranks fine but doesn't start and run, I'm not the expert some people think I am. If it doesn't crank, or it cranks slowly, suspect the battery, cables, or cable connections. I ruled out other parts and circuits because it does crank with a jump-start. That indicates the voltage getting to the starter motor is too low and the jump-start vehicle is raising that voltage up.

If you do indeed have a no-crank condition, turn on the head lights to cause some current to flow. That will make the tests more accurate. Use an inexpensive digital voltmeter to measure the battery voltage. It should be 12.6 volts if the battery is fully charged. Put the meter probes right on the battery posts, not the cable clamps. NOW move one meter probe from the post to its cable clamp. You should see exactly the same reading. This page will describe it better and it will explain what you're looking for:

http://randysrepairshop.net/voltage-drop-tests-in-a-high-current-circuit.html

You're looking for a "voltage drop" which indicates the presence of a bad connection, but the voltages you measure are directly related to how much current is trying to flow in the circuit. Turning on the head lights is only useful for checking the battery cable connections, and nothing else in the starting circuit, and it will not be as accurate. Still, it's a starting point.

Your jumper cables might also be squeezing a cable connection so it makes better contact with your battery. If that is the case, try connecting just one cable at a time. The boosting car is out of the picture then, so if that works, it has to be a bad cable connection.