Memento Problem Solved
Thanks to Jason I discovered I had a number of old extensions which were conflicting with each other in CS5 - I removed those and updated a few others and the problem was resolved.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.