My Harmon/Kardon AVR 310 keeps going into protect mode, HELP!?

I recently upgraded my DVD player to an HDMI using the optical cable to transmit audio to my H&K 310. Now whenever I play a Dolby Digital or DTS encoded movie my H&K trips and shuts down! I did not have this problem with my old SONY progressive scan utilizing the optical cable. The receiver is about 5 years old. Any help would be greatly appreciated as I don't want to scrap this unit.