Seamless mode bug?

[Cardcaptor Stacey]

I tried out seamless mode yesterday I was completely blown away by how brilliant it integrated with my desktop. Unfortunately though, after a little while of inactivity, VirtualBox sucked up all my CPU and my guest OS became unresponsive. After sending the shutdown signal many times I managed to turn it off. After booting back up, seamless mode has never worked the same again.

When I switch to seamless mode, a small part of my guest OS flashes in the top left hand corner but then disappears completely. I then see nothing and therefore can't interact with it. When I switch back, it goes back to window without any problems.

Here's my set up:

VirtualBox: 3.1.4
Guest OS: Kubuntu 9.10
Base memory: 1024MB (I have been playing about with this to see if it makes a difference. I have plenty more memory to share)
Video memory: 128MB
3D acceleration: Disabled
2D video acceleration: Disabled

I have guest additions installed