Well, I took your advice and swapped cables, twice. Also, if I go direct with one HDMI cable to the projector I can re-set the resolution without any problem but once the switch is involved, it's no go, even after the cable swapping.
I don't think it's the switch (a Monoprice powered switch) as the other sources are fine through the switch but the PCH remains problematic.
I actually managed at one point to get it to accept 1080p 24 through the switch but as soon as I tried to play an avi file (i,.e. a non-1080p file) it gave me a green screen and then a black one, eventually saying "no signal". I just tried to re-set now using "TV mode"+7 to change it back to 720p but the screen now says "unsupported" with different H and V values every few seconds.....
I'm wondering if the mobo is acting funky.....I'm inclined to get a WD TV Live box tomorrow and try it out to see if that works any better.