Tivo premier xl and xfinity on demand probelms

For the last several months our on demand has been very sporadic. We can sometimes watch a program but mostly get error messages with csm-11 and csm-12 codes. We have tried rebooting and sometimes that works for a little while and others not. We've called tivo and comcast repeatedly and even had a tech from comcast out. Tivo says it's a comcast problem and comcast says it's a tivo problem.