I have this machine, first gave a SC605, and after a Memory all clear It start to copy some pages.
In one occasion SC605 and in two occasions SC991.
1) After few copies, It hang on and said "copying" in the display. It never get out of this condition.
2) Turn it off and on, start copy some pages, hang on "copying" again.
3) When machine hang I try to do some SC but it doesn't work it just said "executing" forever. (Hang)
4) When on and off again, I enter in Service Mode all SC modes work perfect.