Unique problem - one channel silent under different variables

Bought a mint (looking) Cary slp 98 on eBay, connected to two reconditioned Hatachi HMA 7500 mosfet amps, work perfectly for years since reconditioning - biamping NHT 4 towers, bought new. 
When the preamp is turned on, a series of pops through the speakers, then Only the right channel plays. Switched tubes, didn't solve issue. Changed inputs from preamp to left and right amp and immediately both channels worked. Switched off, waited 5 minutes and still both channels working. Came back a few hours later, switched on again (having reversed the inputs to normal) and again same sequence: left channel silent and after switching inputs, both work and remain working until I switch off and come back when the units are cold.
I can understand that that there may be a resistor/capacitor that need to be hot before working, but why would switching input cables from left out to right input of amp and vice versa immediately fix it!
so every time I want to hear music I have to go thro this rigmarole! Why does switching cables fix the problem????
any solution will be greatly appreciated! I'm sure it will end up back at Cary but I wondered  if anyone (Almarg???) has an answer?  Does the popping on switching on, both channels pop, provide a clue?

Might be a problem with the left channel amp. Swap the left and right speaker cables and see if the trouble moves to the right channel.
Thanks for the clarifications, Denis.  Apologies if I inferred too much from your initial post (which I did). 
I have found that if I leave the preamp on constant standby, the issue is resolved.
That's obviously a key finding, but again it doesn't point to a specific cause as far as I can tell.  But have you done enough experimentation to be certain that what fixes the problem during the minutes following a cold start is switching the cables, as opposed to just disconnecting and reconnecting them, or not even doing that but simply cycling the power at the corresponding time intervals?

Best regards,
-- Al
Yogiboy, I think you're right, because my last preamp was silent on left side and is currently being checked. Never thought it could be the amp - could transients and "pops" emanate from and cause a ss mosfet amp to shut down? How?

I will do the experiment.
Denis, I found a thread from about two years ago at another forum in which the OP had a problem with an intermittent popping noise in one channel of a Cary SLI-80 integrated amp. That amp was introduced in approximately the same timeframe as your SLP-98 preamp.

In that case the problem turned out to be caused by a bad Jensen copper and oil capacitor. It was mentioned by several people that those capacitors had high failure rates in times past, apparently including at least some of the years in which those components were manufactured. It was also said that Jensen confirmed that they had had a bad run of those capacitors.

I was thinking that a bad capacitor might be consistent with the symptoms you’ve described even before finding that thread. It might pay to have a technician (not necessarily a Cary person) replace all of the Jensen caps in your unit.

Good luck. Regards,
-- Al

If swapping the cables left for right causes the problem to go away, the problem is unlikely to be the preamp or amp. Its far more likely to be a failing connection in one of the interconnect cables. The act of switching it to the other channel might be putting some strain or removing some strain in such a way that the broken connection can make a mechanical contact.

 While it is true that the coupling caps could be having a problem, its not been explained how this is alleviated by swapping channels! So IMO/IME the cables themselves are the first thing to check.