When you have an intermittent problem you have to test the charging system while the problem is occurring. The most common cause on Chrysler products is worn brushes inside the alternator. They can be replaced for nine dollars. On some car models you don't even have to take the alternator off the engine.
If you want to verify this is the problem, measure the voltages on the two smaller wires on the back of the alternator while the engine is running and the no-charge is occurring. You'll find full battery voltage on one wire. The other one must have less but not 0 volts. If you find 0 volts, replace the brush assembly.