Desktop background problems: 27" iMac/Mission Control

I've just installed a 27" iMac running 10.8 with 16 GB RAM. I use 4 desktops at a time with different backgrounds in each. When changing between e.g. desktops 2 and 3, via CTRL-right-arrow, Mission Control annoyingly flashes the desktop 1 background before replacing it with the desktop 3 background.

I have replicated the problem on the other 27" iMac's I have access to, and with a variety of desktop backgrounds (e.g. only the Apple provided ones)

and with various amounts of RAM (8 GB to 16GB). It is a consistent problem on all 27" iMac's running 10.8 as far as I can see.

Mission Control handled multiple desktop backgrounds flawlessly under my previous set up (iMac 21"/10.7).

Does anyone know how to fix this behavior?