Screen sharing using high CPU!

Hi

I have a strange issue on my Mac Mini.

I run it without a monitor as it is primarily an iTunes server but mount it via Screen sharing to my iMac.

Just FYI I am also running iStat Menu's so can easily see CPU performance etc.

If no activity on the Mac Mini everything is fine and CPU utilisation is quite low, apart from some BOINC apps that are running.

However, if there has been, not currently but has been some screen activity on teh Mac Mini then CPU is running about 60% and this is almost totally Screen Sharing!

If I 'touch' the iStat CPU menu, this high CPU falls away back to normal levels.

Something really weird is happening and i do not know what it is.

Any ideas?

Robin