Give more details on your config and observations.
When you create the sequence and it has 500 channels, do all 500 channels flicker?
Do they flicker only during the playback? Do they flicker when you F3 test an effect with Output to lights enabled?
My RGB lights were doing a very strange flicker after several months of good testing. It was the new Belkin switch that was the issue. I power cycled the switch and the flicker went away.