Oncidium with yellow leaves but new growth?

[bergdj]

So I left for a couple of days and when i got back i noticed 2 new yellow leaves on my oncidium. I've been having some trouble with this plant and I'm not totally sure what to do about it. Is it normal for them to yellow and drop leaves? Mine has dropped a few since I got it.

That said i've been noticing a few of the leaves growing well over the last couple weeks. Because of the yellow leaves I was giving it a good once over tonight and noticed 2 new Pbulb growths (i think). Can someone tell me if this is normal growth or if I should be concerned about the yellowing and falling leaves. I've attached some photos. In the third one you can see the new growth well as well as the old shriveled Pbulb. Any and all advice would be awesome.