Hi Stephen,
I've had my fair share of phals that looked just like yours. I babied them til no end and they did not improve with time, for years in fact!
Like you said ... they turn yellow and die.
They can come back (mine did) just to regrow, turn yellow and die again.
No one that I have come across seems to know what this is. Mesophyll cell collapse tends to come up alot.
In the end I just tossed mine out - scared of spreading to my other orchids. These we're just to questionable in my mind to keep.
I'm sorry I wasn't more encouraging to you
Hopefully someone else maybe able to shed some light on this subject.