You are in the same boat as me. People say this problem existed before 269, but I never experienced it until 269. I had my box replaced also and it continues on OTA, SD, and HD channels. My dolby digital feed is flaking out more than before too (2 seconds or so at a time of no sound, receiver goes back to prologic, then back to DD, this does not happen with my other receiver nor when playing DVD's).
A friend who has an 811 and 921 notices it also on his 811 since the 269 update. We both get it on an infrequent basis after about 30 minutes of watching a channel, and the only easy fix is to change the channel and come back and it's fine. But it happens way to often for it to be just a random problem.