Left channel fading in and out

You don't say, but I assume the system was working before and this is a new problem. I would first check your TT setup to make sure that your cart didn't get knocked out of alignment, or something similar. If the problem is still there, I would start with swapping components, one at a time, until you find the problem. Do the easiest ones first, like cables.

Great advice from Zd542! I had a similar problem.Turned out to be the power switch on my tube preamp.Good luck,Be careful not to short anything out when troubleshooting.To be on the safe side ,turn everything off when changing cables,tubes ,and let us know how it goes.

Marek, after checking the turntable as Zd suggested, what you should do is to interchange the left and right channel connections at various points in the signal path, perhaps starting somewhere in the middle (e.g., at the outputs of the preamp). If the problem continues to be in the left channel, you will have established that it is being caused by something downstream of the point where the interchange was made (e.g., the amplifier). If it moves to the other channel, you will have established that it is being caused by something upstream of that point. Continue that process until you identify the component or cable that is causing the problem.

As Ray indicated, of course, shut down the system before changing any connections.

Best regards,
-- Al

What Almarg said.
With reference to Zd's idea, I cannot imagine how "knocking" the cartridge out of alignment could produce the phenomenon that you described, but you're going to find out whether the cartridge is the cause by swapping channels all the way up the chain.

Unfortunately, IME, when one has an intermittent problem such as this it typically fails to reveal itself when one is hunting for it. I have been trying to find out for 8 months what is wrong with my Sony CDP. It malfunctions in my house but not on Bill Thalmann's workbench, except for a brief moment which allowed us to confirm that I am not imagining things.

Lewm,

"I cannot imagine how "knocking" the cartridge out of alignment could produce the phenomenon that you described."

It can easily do that. The groove in a record is shaped like a V. The info for the right and left channels are on each side of the wall. If the cart is not aligned properly, it may not be fully able to touch both walls the amount it needs to produce full resolution, causing an intermittent problem. I also said "or something similar". Things like not having enough tracking force, anti skating and SRA, can also contribute as well. If the problem is alignment related, it can very well be a combination of the above. Or, like you say, it may be something else entirely.

As far as your CD player goes, that seems like an interesting problem, as well. I think you need to find out what is different about Bill Thalmann's workbench than your equipment rack. (I know that sounds like a smart ass remark, but its not.) There really may be some difference you are not aware of that is causing the problem. It could even be temperature related. Just by taking it out of your system for a while may let it cool long enough for to work.