Sudden crashes and drops in battery charge
You might need to reset the battery stats anyways.  Drain the phone entirely -- when it shuts off, turn it on again until it shuts off again and repeat until it won't turn on.  Then wipe the stats and charge to 100% with the phone off (leave it plugged in for an hour or so after it appears to be fully charged).  Turn it on, drain it again, and charge to 100% while off again without wiping the stats this time.

Of course, if you're not rooted you can't wipe the stats, but the drain/recharge cycles should help regardless by providing fresh stats.