RE: CODE P0622 and VOLTAGE LIGHT ON
An open or short in the field control circuit. It could be wire, the alt, or the PCM. If the condition goes away after running a bit (more than likley heat related), I'd go with one of the two components (alt/PCM). The alt is cheeper and easier to change so...I'd try that. Now if you are handy with a meter, go to the top of the sec gen fourm and you will find the "holy grail". Use the info to aid in troubleshooting.
You could also go to Auto Zone, and they can test your charging system in the car for free, however, this specific malfunction may not show up, as it's more of an optimimum charge issue.