T430s Screen flicker issue - any ideas?

[starsknight]

I've had a T430s for about 4 or 5 years now, and on the whole it's performed well. Over the past few months, though, the screen will suddenly dim for about a second, then immediately return to its former brightness. I just replaced the hard drive and did a fresh install of Windows (7 Pro), and the problem's continued, so I'm wondering if it's a hardware issue . . . don't know of anything that would cause this, though. It happens regardless of whether the computer's plugged in, and it's not an autodim due to inactivity issue--it happens while I'm typing.

I've done some Google searches to no avail. Any ideas?

Thanks in advance!