If I had to guess, I'd probably point the finger at the cloud provider...
There's nothing in the apache/tomcat stack that would inherently cause performance degradation over time. The cloud provider, on the other hand, is probably juggling resources to make highly-accessed resources more available then lightly-used ones. It can determine, over time, which resources are hot and which resources are cold, and may be the one that is interfering w/ your performance metrics...
