Regarding the reboot : Imagine you drive 100miles at 100m/h. At soem point you increase your sepped till 150 and then back to 100. The chanses that the increase in speed will affect your avg speed is very little. Same with the reboot. At some point the VM's memory use will "settle down" again.
This is waht I would do. Bring the memory doen to say 6gb and monitor again. also monitor the Stress badge as this will eb you indication if there is a hogh memory demand.
Read my article : http://www.virtualclouds.co.za/?p=544. Should explain this a bit more.
Hugo