Is my phono stage stuffed

Hi. I have a MF NuVista M3 Integrated amplifier. As of yesterday the left channel of its phone stage has dropped out either altogether and just a emitting a faint scratching sound and occassionally it comes back to normal life but for a minute at most.

I believe it to be the left channel of the phone stage because:

1) I get sound out of both speakers when the source is switched to CD;

2) I switched the tone arm cable plugs at the phono stage RCA input terminals - left with the right and vice versa and still there is no output of the left speaker.

3) I double checked the grounding of the tone arm to the grounding of the amplifier. At a very low minimum volume I induced the typical "hum" generated by not grounding but the hum only occurred in the right speaker not the left.

Any ideas anyone? anything else I should check test. Unfortunately I have no other phono stage to check if everything else is working as it should be.

cheers