I also have the same intermittent flickering of the dashboard lighting for tailights. My parents also own the 2000 RX300 and they have the same problem. After seeing your post, I now believe this is a defect by Lexus.
I loved my RX300, it has been the best vehicle. I have been impressed with the dealers service. I did have the key problem and was able to fix it with the dealer fix kit, at a reasonable price. Basically all I had to do was brakes and tires with this vehicle. I bought this car used at 43,000 miles, and sold it at 125,000 miles and was sad to sell it, it was a great car for me. I have bought another Lexus SUV used.
I purchased a 2000 RX 300 with 42K miles and I loved it. I maintained it religiously, never drove the vehicle hard, and changed the transmission fluid when asked to do so by the dealer.
At about 93,000, the tranny started giving me problems. When I took it in, they said I had major sludge/metal slivers in the tranny. I was told it would be 5-8k to replace.
After reading several comments on Lexusownersclub.com, I was convinced that I should dump this "perfect" vehicle. Lexus was not helpful and said they knew of no problems. I read that the problems with the transmissions was due to a design flaw. The transmission fluid was designed to cool by running parallel to the engine coolant in a separate system. NOT a very good way to keep a hot running AWD tranny cool. Toyota improved the design and installed a separate cooling system. From that point, they have had no problems with the RX300/Highlanders...
I bought the RX300 new on July 1, 2000. Now sitting at 132,000 miles and still looks/rides/works great (put 50k of that on in the last 18 months, all on the DC beltway).
I do all my maintenance at local shops, not at the dealer - they are usually 50% to 100% more at the dealer than the local shop for the same work.
Only major issue so far were brakes - had them done at a Midas dealer, it was awful. Took 2 repair shops to do it right, but now better then ever.
Also upgraded the tires to Michelin, made the ride smoother and car more stable.
I change the oil at every 4-6k miles, and it is still very clean at the dip stick, so hoping that no sludge issues for me.
Overall, this has been the best, most reliable car I have had.