Custom SessionState Provider and Web Farm

I am working on extending our web application to work in a web farm. I wrote a custom session state provider. All the parts seem to work on their own, but together the site eventually becomes unresponsive until I clear the session cookie, and then suddenly it all works again.

When I use a custom provider with one webserver everything works fine.

When I use a farm with both webservers pointed to the same SessionStateService, I can bounce in between and stay logged in, etc. as expected.

Once I try to farm against a custom provider it works as expected for 3-5 pages of requests, but eventually it just stops responding and stuck in the browser. Waiting for a page ...

I have installed the machine keys on both machines (verified by the session state service). While this is only true for view state, I believe.

Are there any other settings or something in there that I am missing? This is my only guess at this point, other than the blocking problem, which is one that only shows itself on the farm. Two days of fighting this left me unanswered ...