As far as cutting, as the output coupler gets stressed, you'll usually have to move your focus higher and higher to be able to cut. Or it will cut for a 1-5 minutes, then go bad. On stainless cutting, it may cut OK for a while (1-5 minutes), then you'll lose the cut. Eventually you won't be able to cut at all.
To check it, you can take a cold and hot mode. If the hot mode is getting a lot deeper and diameter getting a lot smaller compared to cold mode, then the output coupler is stressed and needs to be replaced.