It sounds to me like your shifters are slipping, thus allowing the rear derailleur to downshift as the shift levers slowly slip. Correct this by taking one shift lever apart at a time, clean it thoroughly and put it together. Once assembled with your cable fitted and attached to the rear derailleur, ensure that the friction adjustment(the little screw in the middle or each shifter) is tightened up enough to create sufficient friction to prevent further slippage. Hope that is a help.