greesemunkee One thing you might try to just to identify if it is part of the problem is to buy one package of sure gel binder and try that on a batch. Sure gel is a VERY good binder but as you can see it is not cheap, if you wanted to try something else I would go with Super Bind which is a mixture of carrot fiber and potato starch. Using one of these two would pretty much ensure that if a binder was going to fix your issue then one of these would. For the money I like Carrot Fiber best though, inexpensive and does the trick.
As doc_craig says protein extraction (mixing) is also VERY important in preventing the fat from rendering out of the meat which could cause your rubbery casing as the fat will come out of the meat and the casing will be cooking in it during the entire process.