Frames fading when i am working on my project but non of the settings have changed

So I have been having a random problem with animate the last couple days. Some of my frames/layers will randomly change transparence. Suddenly one of my characters heads will be nearly invisible but if i go back a few frames and then back to the frame i was orignally on its fine. Or my background will suddenly disappear, I have no idea what is happening. Non of the settings have changed, alpha levels are still the same and the problem fixes its self shortly and if i test my movie it doesnt change anything there. It seems to be more just an annoyance then anything but I just want to make sure this isnt part of a larger problem that will ruin my project. 

Thanks for any help