I have a TiVO series 3, the sound will drop out for about 5 seconds, the picture freezes for about 1-2 seconds, pixalates, then jumps back into "normal mode". This happens anywhere from every few minutes to several times during an hour. Also, most of the time when a recorded is to start it will tape a blank screen. It's often difficult to get the channel to show. It sometimes can be done by clicking up a few channels, then down a few. This happens on all channels, standard and HD channels. These situations happen on two separate TiVO Series 3 units on different TVs.

So far I've had Comcast out to adjust the signal strength, it now shows a constant value of 93 when recording.
I've switched out the power supply and hard drive (did a SMART test on the hard drive, everything came back ok)
Switched out the cable card.

I don't know what to do to do, suggestions?