Yeah its something to do with multi monitors but happens even if your just using one monitor. If you overclock your idle clocks will be 400Mhz Core and whatever you've overclocked your memory to. With the 10.4's its 157 core and 300 mem.
I went back to 10.4 as I actually lost alittle performance with the 10.5's and I don't want the higher idle clocks (and heat)