Charging Problems

[jhv48]

Here's the problem: Normally, my ammeter gauge registers about an eighth of an inch to the positive side while driving. When the car is off, the needle will rest on the left corner of the zero mark. Yesterday, while driving on the freeway, I noticed the ammeter would behave normally then suddenly move to the off position. The engine continued to run normally. The needle would stay there for a few minutes and then jump to about a quarter inch on the positive side and then slowly drop back to 1/8" on the positive side (normal). Bumps didn't seem to effect the on/off cycle at all. It seems to be doing this at random. I checked the ground wires and the connecting wire. All seems good. Am I just experiencing a failing alternator or is there a wiring problem I should concern myself with.

I've got a Roush 427SR engine with the stock Powermaster one wire chrome alternator. If I need to replace it, which one do I get? Is #170781 the correct one? I have a grooved serpentine belt and replacement alternators seem to all have v-belt pulleys. Do I just swap pulleys with the one I have now? Also, what amp rating is the one that comes with these engines. I don't want to take it off until I have a replacement.