Had something similar on mine. Though not 30-35 flip bad! It took about an hour of steady flipping to drop a bubble. Basically about 250 pages. Most of it got fixed by doing the "Charge to full, drain to warning tango" a couple times as soon as possible. Now I can get roughly 3,000 PDF pages (from the SD card) before it starts getting low. Just using the internal memory about gets about 6,000.
I've also noticed the bigger the card, the lower the battery life seems to be. The same PDFs on a 1gig card uses more juice than a 512meg card to go from start to finish. (But that could just be my unit.)
PDFs and other images (as opposed to text only) also seem to shorten read time.