Krell HTS 7.1 Problem

I am using a Krell HTS 7.1 processor in my HT system. Most of the time it works extremely well. However, occasionally I loose the center and rear surround sound. L+R and subwoofer seem OK. I can restore everything to normal simply by completely powering down the unit and powering back on. I am using an optical link between my Blu-Ray player and the Krell (current firmware, 3.4 I believe). Any idea what is causing this problem? I suspect I will have to send it back to Krell for repair.