1998 Pontiac Grand Prix

For the past couple of months, my clock/radio light, on occasion, will go dim and then brighten back up, dim and brighten back up. This will happen for a few minutes and then stop. No problems for days/weeks even. Then, it will start again. About a month ago I started my car and had no light on the radio at all. After driving for a couple of minutes it came on. All other interior lights were working. I also notice sometimes when I am parking that my headlights will dim and then come back full strength. Like when I'm breaking, shifting from drive to reverse or vise versa is when I notice it. I had my alternator replaced about 4 months ago. Thinking that maybe I got a bad one, I took it to my mechanic who hooked my battery? up to a voltage reader (sorry, I don't know the correct name) and the reading was a steady 14. Any ideas?