A question for anyone who has a level 85 rogue on WoW?

Okay so, I have a level 85 night elf rogue. And I have a problem with instances sometimes. One instance I'll be doing like 11k dmg and other times I'll be doing like 3k. Or I'll get the same one again and do very crappy. I don't know whats going on and its very aggravating. If you want to look at my spec and give me advice my toon is Wickedbabe @ Anvilmar. Thanks :)