Hi Rob,
I think that if you switch between SD and HD channels, it is normal, because your receiver has to lock in the signal from the HDMI audio from these two incompatible audio signals. Many receivers introduced a certain delay between various audio signals and channels, from their HDMI audio output. And also like you said between different video resolutions. Some receivers are faster than others with this. And it also depends of the collaboration of all your other HDMI components in your system.
Another thing too, some receivers remedy to this problem sometimes too by upgrading the firmware, like for example the Yamaha RX-V3900. For more on this, check the review of the ditto RX-V3900 at Secrets of Home Theater and High Fidelity.
I might be wrong, but I believe that's what is happening.
Someone please correct me if I'm wrong.
Bob