do you use the software version 1.3? I have a 5 gig ipod as well, and I was experiencing horrible battery life issues until I upgraded to 1.3. Now my battery life, both playing and standby, is probably better than new (11+ hours play time).
I have a 3rd gen iPod (4 button scroll wheel) and the battery time is great. I've never run it below 50% and I've had it on for 4-5 hours with moderate fidgeting with buttons. Last night I fell asleep with it on and woke up 4 hours later. iPod was off battery life was 4 full bars. ???
If I remember correctly, older iPods came with 30 day or 90 day warranty. So if something is wrong with iPod after that, you are on your own.
So in your case, I really don't know what to say. What I might try is to go to a local Apple Store (if there is one) and talk to someone about it. But in the end, you'll probably have to spend some money.
Apparently, newer iPod has longer warranty. According to http://www.apple.com/ipod/specs.html, iPod now has 1 year warranty. This really annoys me, since I had to spend extra money for replacement policy because warranty on my iPod was so short.