if entire right side is dropping out intermittently, rather than just a single speaker, then it is one of these:
● loose connection or short in wiring behind head unit
● loose connection or short in wiring where it enters amp
● loose connection or short in wiring between head unit and amp
● problem inside right channel of head unit
● problem inside right channel of amp
getting behind head unit is relatively easy, there are several threads/tutorials here, search for them, and start with the FAQ for your generation - you will be removing console cover around gearshift & ashtray area, if memory serves
OK, I see the OP has a 2000 LS400 Platinum SF which, like mine, came with the Nakamichi system.
What's with the right channel of the Nakamichi failing on so many 98-00 LS400's? There have been a bunch reported on this forum over the years including mine. Why doesn't the left channel ever fail?
I remember a thread on this forum a few months ago that speculated that a loose connection in a ribbon cable inside the radio was the problem. I think it had photos.
As I said in previous threads, Factory Stereo Help in Florida "repaired" my 00 LS Nakamichi right channel in 2008 by re-soldering the balance control. At least he thought that was what fixed it - he wasn't 100% positive but the right channel worked fine until fall, 2013 when it started failing intermittently again.
This last time I gave up and had a $350 Kenwood DDX-470 installed at Best Buy using the Nakamichi amp and all the original connectors and speakers. The installer used pin adapters to plug all the Kenwood's wires into the OEM audio harnesses without cutting a single wire. Total cost was $498. There is a post the LS400 audio thread about the install. I especially like the Pandora interface and wireless Bluetooth audio streaming - use those features almost every day.
I hate to suggest it but you might want to give up on your Nakamichi head unit. I can't find anyone who is willing to work on Nakamichi any more including the company in Florida that fixed it last time and the Factory Radio Service in California that added an aux-in to it.
And don't I get 25 cents each time someone posts my head unit removal instructions? LOL