Is it an intermittent front right channel? Assuming you've tried different sources, interconnects, and speaker/speaker wire, to answer your question, you would need to see if the problem is in the pre-amp or amplifier stage. If it is the front right channel, an easy way to check this is by plugging into the headphone output . If everything works fine, the problem is almost certainly in the amplifier section and inputting a signal from another pre-amp would not be a viable work around.
Feel free to email me, at firstname.lastname@example.org, with your results so that I can be of further assistance.