I had the same problem where my altitude seemed to rise and fall by about 1000ft. The engines were erratic and if I touched the throttles the engines would either increase or decrease. I turned all A/P VNAV LNAV and autothrottles off and then re-engaged everything as well as making sure that my joystick throttle was in the idle position. This seemed to settle everything down as long as I did not touch the throttles.