Default [Q] Volume goes up automatically, power button doesnt work

I have a Blue Angel (XDA IIS) that has been flashed with WM6.1. I have been using it for a few years until recently that I got this weird problem.

While the phone is idle, suddenly the volume increases automatically by itself, as if some one pushes the volume slider up, and keeps pushing it. As a result, the speaker keeps beeping as the volume already hitting the max.

After awhile, it will then stop. For a short while later, it starts again. While this is happening, I cant turn off the phone using the power button on the top. But I can turn off the backlight by doing a long-press.

I have tried to reflash the firmware and also did a hard reset, but it doesnt solve the problem.

Anyone has encountered this weird problem? The buttons seem to work fine when not in that weird condition. If it is not software problem (since I reflashed it) and hardware (buttons work fine in normal situation), what else could it be?

Any idea anyone? Really appreciate some advice.