I am also having this problem when playing back either SD or HD content that has been recorded on the non-pvr set.
It seems to be a bandwidth/buffering problem. At one point durring playback last night it actually said that the setup box had lost its network connection.
Pausing for a few seconds then resuming seems to solve the issue temporarily. I am fairly sure this is due to a wiring issue as both my PVR and non-PVR box were put on the existing coxial cable instead of having the PVR on CAT5e.