video and audio problems !!

[hammersmith]

i used to have a HK receiver and everything worked great.
then i got a yamaha 5180 receiver and i have problems.
same sources . . . popcorn hour a100 and a xbox 360.
the only video setting the pch works on is "auto." then it defaults to 720p.
my tv works at 1080p but when i set it to this, or 720p, the tv says no signal.
i have to reset it to auto, but like i said, that only works at 720p.
the xbox also outputs at 720p . . . it used to output at 1080p with the HK.
also the audio signal it sends through hdmi doesn't use the center channel unless the source is ac3.
it doesn't seem to matter if my settings are bitstream for all for options or bitstream for dts and ac3 with pcm for the other two.
what should my settings be??
why doesn't it work when i set the pch to 720p or 1080p?

thanks