Sounds to me like you phone is using more juice than you're putting in... Maybe you're using a 'slow' charger? If the cable is damaged this may reduce the current output, so your phone will be slower to charge. If you're using it while charging and this happens, sometimes the use is bigger than the input, so it will look like your battery is actually depleting (which would go down faster if you were using it and not charging it).
Another thing you could try is to re-calibrate your battery. Charge it to a 100% and then let it go down to zero (well, not actually zero, but until the phone shuts off by itself) in a single use; this is, don't charge it mid-day, let it go out entirely. Then charge the battery again to 100% (again, don't interrupt the charge, let it go to 100% on its own). This should give the phone a better battery level reading.