waveform on Rigor 1054z in Average acquire mode drifts to zero

[SW_Perspective]

I just got a Rigol 1054z and am learning to use it. I was using it with simple RC circuits driven with a sine wave from a function generator, using channels 1 and 2 and I adjusted the position of the signals to be superimposed on each other. Acquire mode was set to Average. So far, so good. I measured Vpp and the results were about expected. Then the waveforms began to to flatten out and drift to zero. Changing a vertical scale on either channel or the horizontal scale momentarily changed the waves back to normal and then slowly they would fade toward zero. Changing the Acquire mode to any other setting brought the waveforms back to expected size and there was no fade. And this problem is intermittent. When connected directly to the function generator signal fade did not occur. Until I tried again a bit later when signal fade occurred in this configuration. As I'm just learning I thought I must have set something wrong on the scope or maybe the the circuits had problems with their construction (altho one resistor and one capacitor on a breadboard that seemed unlikely).  Is this a problem with the scope or am I just not smart enough to use an oscilloscope? Is there something I can try to test the scope? Is this a problem anyone has heard about before? I've searched on-line for answers without success. Please help.

[ataradov]

Is your signal properly triggered before you enable averaging? Average of sine waves with a random phase is 0.

Also, what do you mean by "connected directly"? How else do you connect it?

[Mechatrommer]

 Make sure triggered signal swing is not too small on the screen and set trigger level to the center of the signal... you have intermitent trigger there, ie trigger level is near either the min edge o max of the signal.. activating auto (random) mode from behind the average screen. thats why average went to 0.. anyway, set to average by beginner is not a good idea.. real measurement only use peak,normal o high res. Average is only when you know what you are doing and what its for... not a daily life "i think its good" usage, it is not...