What are a few things that would set off this indicator light?

I was driving into work this morning and all of a sudden it came on. Whe I hit freeway speeds it dimmed off went off then came back on.

I pulled over to check, battery connections were fine, and the alt belt seemed okay. I drove off and it went out for the rest of the ride. Then about a block from work it came back on.

Any ideas? Thanks. Oh and the engine was fully rebuilt recently.

 

usually means the alternator is going out, have a shop check the charging system to see what voltage it cranks out under load.

 

Okay cool, cause I have a spare from my SOHC ZC swap. I think we reused mine.