What resolution are you running when watching Blu-rays? It may be that you are using more bandwidth on the HDMI cable with the Blu-ray, and the extreme length of your run is producing the occasional drop-out. It seems odd, however, that you don't get other errors (such as the white specks often called "sparklies") if that is the case.

Can you swap places between the HD DVR and the PS3 so that the PS3 has the straight shot to the TV?