Not to sound harsh, but either your tank isn't tanking right or your dps is spamming too much and is stealing aggro. I've never had problems with the Thief class like that, ever. In fact I've duoed/trioed instances where three and five man teams were needed, respectively. If you build it right, Thief can tank, as well. It's a pretty well-rounded class. You just have to invest in the right skills and know what skills to use when.