electrical question

[MNYPNNY]

I have something strange happening with Penny.
I noticed a few days ago that the lights behind the gear markers in the dash were flickering. Didn't really think anything of it. This morning I drove in the dark (that rarely happens) and discovered that all my lights are flickering. Dash lights, dome light, and headlights. It takes me about 15-20 minutes to drive to work. By the time I got there, the flickering had ceased.
I took Penny to my shop this afternoon and had them check the battery and alternator, both checked out ok, although the alternator is running a bit low.
But, I had driven the 15-20 minutes back to my mechanic from work and the flickering had ceased when the guy was checking it. Almost as if the battery had fully charged while driving for that amount of time.
The terminals are kinda corroded and they suggested that I clean them off, which I will do.
Anyone have any other insight?

~~Michelle