virtualbox.org

I am running VB 4.2.18 on Windows 7 host. Guest OS is Ubuntu 12.04 LTS.

I have assigned 2 GB of RAM and 4 virtual CPUs (Intel i5). Whenever I got to the full screen mode, the windows freezes and takes around a minute to open even terminal. But if I come back from the full screen mode everything becomes normal as if there was no problem.

After disabling 3D acceleration, the full screen mode has no freezing issues however there is certainly some compromize on the amount of animation.

What could be the reason. Is anyone facing this issue. Will VB4.3 beta will fix these issues?

First I would only allow 2 CPUs at the most and next you need to see if the guest is really using the OpenGL drivers from VirtualBox.
With 3D enabled run the following in a terminal Post the results

also the following:
From the hosts terminal/command window type VBoxManage showvminfo <VM Name> --details and post here. Replace <VM Name> with the actual name of the VM. Use " " if the name has a space in it.

Note if the host is Windows you need to be in the directory that has VBoxManage.exe for the command to work.

and post the guests log file (as an attachment)
See Online Users Manual for the location of the file.