Stereo mode in phone stage going out

Forgot to mention, do you have this problem when switched to any of the high level inputs?  For example, CDP or tuner?  The stereo/mono switch is typically in the linestage circuit, not in the phono stage circuit, in an integrated amp or receiver. By that reasoning, the loss of a stereo signal ought to be happening  when you are listening to any source component, if the switch is at fault. 

Thanks for all the suggestions.

Over the past week, when playing vinyl, usually about the 4th/5th song is when the (always) right channel goes about 98% out in stereo mode. Flipping it to Mono restores the sound in both channels. 

All other programs (CD, Tuner, Streaming device connected via Aux) work fine in stereo mode. Never a problem there.

I have not connected the TT to Phono 2 but will try that.

The TT is a Rega 1, four years old with a 2-year-old Ortofon Blue cart. 

Considering what you indicated in your last post, I suspect it is a dirty, intermittent contact in the source selector rotary switch.  The only thing that has been moved is the selector when you changed from your previous input to phono.  Those rotary wafer switches are complicated little buggers with lots of contacts. When you lose your right channel playing LPs, try rotating the input selector switch back and forth and see if the right channel returns. That would likely indicate dirty contacts in the selector that would need cleaning, or perhaps a failing solder connection where the rotary switch is soldered to the PC board.  Both easy fixes by your serviceman.

I had a sx1250. It’s the stereo/mono switch. 
you need some deoxit to clean it. Then you should be good.