I also have a 1999 LS 400 and my dash lights dim or flicker when I am driving, but after awhile I was able to narrow it down to when I go under overpasses or enter tunnels.
Like you, at first I thought this might be leading to the instrumental panel failure that some of the 1st gens have, but after doing some studying I have read that these gauges adjust their brightness level to the ambient light conditions. I have verified that this is true, as soon as I enter my garage the lights go dim, when I pull out they get brighter. A neat feature in theory, but a little aggravating sometimes on the highway.