Yamaha R-300 left channel intermittent
I just picked up a Yamaha Natural Sound Stereo Receiver R-300. I was testing all the knobs and buttons when I pushed the stereo/mono button the left channel went out in the stereo position. I was playing a MP3 player through the AUX input I reinserted the cable and it was fine, last night the speaker was on and off. This evening I had it on for an hour and it was fine. Any suggestions on where I should start and what products I should use for cleaning switches and or connections? Thank you in advance.