There may be an issue with the gas gauge line. This is typical in iPhones where a small filter gets dislodged from the logic board. This filter allows the continuity of this circuit and if it is damaged, the CPU has no real idea what the actual battery level is. The iPod has fundamentally the same circuit design
Was this fluctuation occurring before the battery replacement?
It is possible you have a defective replacement battery and depending on where you bought it, you may want to consider a high quality [product|IF119-001-2|replacement] instead. Otherwise, you will have to have this looked at by a micro-solderer.