Rotel RMB 1075 drops left channel


I just got a used Rotel RMB 1075 power amp and am finding that it drops the left channel repeatedly. The loss only lasts a few moments and it kicks back in.

Although the unit is 5 channel, I only set up two initially, so I don't know if it drops other channels or not.

Any suggestions as to what might be the cause?
rrichmo