Originally Posted by KCBrew
Do you ever add straight tap water to the beer? Is it possible that you top off post boil? Perhaps your municipal water supply doesn't have consistent additions of choline/chloramines to the water supply?
The inconsistent levels of chloramines or chlorine are very likely. I've read that every so often a water department may decide to flush the system with higher than normal amounts of these chemicals. And I remember one time water from the tap reeked of chlorine. I called the water department and they told me they had accidentally let too much into the system.
When I started using campden tables, I noticed my beer tasted cleaner. I don't know that I would have called the off flavor 'rubber', but there was a definite difference.
Try campden tablets or a charcoal filter and let us know how it goes!