That may be the sign of a dying battery. Even though its fully charged the battery is unpredictable, and will provide under/overvoltage to the device. My old Curve used to do the same thing until I replaced the battery. My Galaxy S2 was doing that earlier this year too.
I tried a battery off eBay but it was junk. I later ordered a battery through these guys while ordering my wife a new laptop battery, and it is as good as new now.
http://www.battdepot.com/ca/
Might be worth trying for $10 before paying to have someone look at it. The 9900 is likely 2+ years old, so its probably due.