The amp gauge in my truck has been abruptly falling to zero when the engine is running I put a voltage meter on the battery when this happened and the volt were 14.34 engine running I have just replaced the battery could the alternator be failing or the voltage regulator when above occurs if I turn the ingnition off for a min and then restart the amp gauge is normal but seems to fail after a few min and falls off to zero

Sounds normal, to me, if I have the correct picture of your situation. After you start your truck, the ammeter likely shows that it's discharging; then, as you run the engine, the ammeter cuts back to zero, indicating everything is "steady-state."