musical fidelity nu-vista

been along time since i was on here and was hoping to get some advice.The left channel has started to cut in and out and then finally stop working. This happens anywhere from 5 min to 30 min of use at moderate volumes. i switched over cables, slow blow fuses and the tubes and still have the same problem. i have very limited knowledge on electronics..any ideas what the problem could be or what i should look at?

thanks
glen
 
heres a pic
 

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