Help! oncidium roots with black spots...

[Zekeyflower]

I have an oncidium orchid that might be dying. A little over a month ago I repotted it in spaghnum moss/bark mixture- it used to just be in bark. Then I started noticing little gnats flying around the plant so I bought some of those sticky traps and I haven't seen them since. But just today I noticed a couple of the leaves from the newest pseudobulbs are turning yellow at the tips. I freaked out thinking that they are infected and maybe the gnats are still in there. I unpotted it and did not find any evidence of pests but the roots look bad. Most of them are brown and the ones that are green/white have black spots on them. The roots are not mushy- even the brown ones feel firm. So my question is, do I cut the roots off or leave them? Are the black ones infected? Im planning on repotting it in just bark again because I think there was too much moisture with the sphagnum moss.... I just don't know how many roots I should be cutting off. Any help would be greatly appreciated cuz I'm clueless.