Blackberry power issues

[Agrippa]

I have an 8700g running OS 4.2. No power issues (battery will drain normally) when device is idle or I am using it only to read/write pushed e-mail. However, even when fully recharged, after a few minutes of using an application that connects to the Internet (such as Google Maps, Opera Mini or the native web browser), the device will power down as if the battery is dead. The device will boot up if I remove and re-insert the battery, but will immediately power down again once the desktop is loaded (sometimes I catch a glimpse of the red text warning that battery power is too low). But if I reboot while connected to a power cord, the device will start properly and work as described above, even after being disconnected from the power cord.

This is not a battery issue, as I obtained a new battery and the same problem occurs. The representative at the T-Mobile shop pointed out that my water damage indicator is red (I do not know how or when it got that way) and suggests that I may have irreparably damaged the device.

Any ideas? Is it time to get a Curve? (I had been hoping to wait for the 8900.) Many thanks in advance.