EAR phono pre problem ??? Help needed..........

I've ran into a problem playing vinyl the last couple days. Everything will be sounding great then my left channel just stops! No sound but the right is still there. I noticed after turning the EAR 834p off and on again. The sound is restored and lasts another forty minutes or so. Then the same channel shuts off again. Dido and the sound returns. Have I a tube that is getting over heated and going down? What do I do ?? Help please.................