AltHold problem

On my DIY NightHawk clone (EMax 2204-2300kv motors, Afro 20A ESC flashed with BLHeli, 6045 props, ardupilot 2.6 with GPS) I tune PID, copter response is very good, but when I switch to AltHold mode it start steady descend while shaking up and down. I look at the log (attached) and find normal throttle in but very oscillating Throttle out (actually not really oscillating but erratic up and down). The only one other strange thing is the AccelZ a bit high and all negative. If AccelX and Accel Y are within +2 to -2, the Accel Z fluctuates between -5 and -15.
Any idea what may cause it?