Voltages say your battery and charging system are fine. You say when you're able to start the car with the key the ammeter reads high—do I understand you correctly that if you don't use the key to start the car, the ammeter reads normally? If so, your symptoms suggest an internally faulty ignition switch. Don't know if you've ever tried it, but if you're driving down the road (car in gear, engine running, duh) and you turn the key to the "Start" position, the ammeter will peg high. Try another experiment: the next time the engine's running and the ammeter reads high, rev the engine up high enough to allow you turn the key off and then back on without the engine stopping. Try it several times in a row if necessary and see if you're able to get the ammeter to come down to normal reading. If so, replace the ignition switch.