Test the suspect battery with a hydrometer.... Sounds like one of the cells might be dead or its low on fluid caused by the heat of the engine. Fill with distilled water! The condition will only get worse with time if its a dead cell.
Dang man, you go back a looooooong ways. I haven't seen a motorcycle battery with accessable cells since,,,,,,,,,,,,,,, oh, I don't know,,,,,,,,,,,,1962?
If your battery is not maintaining a charge it's most likely due to a weak, (Not dead) cell. I'd certainly agree with Brendan, buy a new battery. However, you could have a drain on the battery causing the problem as well? Maybe an anti theft device or???
The only real way to test your battery is with a load test. Many of the more expensive battery chargers come with a load tester. If you don't have one, take the battery out, charge it fully and stop in at your friendly Les Schwab store and they will test it for you.