Sony Vaio PCG-5K1L Power Problem? Please help

I have a problem with my Sony Vaio laptop (PCG-5K1L, model number VGN-CR320E) that I think is moving from annoying to serious.

This laptop has a little underlight thing which lights up if the system turns on or off, or if it switches from battery to cord power.

About two or three months ago, I'd notice that light flashing for no reason. I then looked on the screen and the icon that would show whether it was plugged in or running on battery power was flickering back and forth. The problem was intermittent at first. In the past month, it's started happening every session.

Today, I noticed that the battery charge has gone down while it is doing this. Normally, when this would all happen before, the battery power would still always say it was at 100%. I've had it on for about 4 hours now, and it's dipped as low as 89%. At the moment, it's back up to 93%.

I don't think the problem is the power cord. It's securely plugged in, the light is on the power brick and the power brick is only slightly warm to the touch.

So what could it be?

I have had this laptop for about 3 years. (I was almost wondering if the battery itself was dying, and sending off these weird signals.)

Any help or feedback would be greatly appreciated! (Please tell me it doesn't mean an expensive repair or new laptop - I just got cut back to part-time. Xl)