Help Battery Issues in Gingerbread (2.3.5)

[Spades Slick]

I recently flashed my phone to install the 2.3.5 version of the OS (using a stock ROM, so all the bloatware is on there). It works fine, except for an intermittent issue I have with the battery. Every so often, after unplugging my phone from an outlet while on full charge, both the OS and Battery Mix will report that I have less than 100%... and the amount it reports can vary from 95% to 15% (I was shocked to hear the "plug me in!" sound when I unplugged it yesterday). This started almost immediately after flashing the rom, before I had installed any of my old apps (the exception being Battery Mix, which I installed first).

Is this a known issue, and is there any way to deal with it other than plugging my phone in again and hoping the charge "takes"?

Thanks.