IMG, there are (3) reels of the same size, the 550SS, the 5500ss and the latest 550SSG.
The 550SS and the 5500SS share the same dogs, the 550SSG has a A/R bearing.
Personally, I'm not crazy about the 550SSG.
There has been some design changes with the dog, you would be OK with either one. I recommend this one, the older style:http://store.scottsbt.com/4-450-Dog-with-spring-fingers-old-style-P779C343.aspx
Bryan also has a good point, I'm betting that it is your dog though. Sometimes the dog appears to be in good shape, but the tip of the finger springs get worn on the inside, cousing the dog to slip every now and then.
The 550ss is a nice little reel, I use these a lot.