1998 Toyota Avalon XLS brake light goes on/off?

My wife just said her brake light had been going on/off at random times though the brakes function. Today her battery all of a sudden went dead, but AAA said the alternator is charging fine and the battery was just little low on a charge. It jumped just fine and has been fine ever since. It's a 60 month toyota battery and only 3 years old. The alternator has been replaced about 30-40,000 miles ago because it was causing the ABS light to come on and the ABS system has already been replaced. Any ideas on what could be causing this and where to start? Could it be the alternator again? The ABS light is not coming on, only the break light.

Thanks so much for your reply!
Rick

Thought everyone should know about a solution to this brake light issue. After further investigation it looks like her brake fluid was just above the min level causing the brake light to turn on when using brakes. I did not know the '98 Avalon XLS had this technology to tell you when the fluid was low. So far no brake light in the past 48 hours once the fluid was increased to the max level.