I am also facing the same issue, I have 2 controllers (c5210) and i have set failover (10sec) and AP poll timer (20 sec).
I see "tunnel breaking, radios going off sometimes, and most of the logs show poll timouts".
I want to disable the fast fail-over, if I disable what should the AP poll time set to.(default / best value) and also is there any recommended MTU value for 3805i's