I have a big problem with Virtualbox.

I had installed ubuntu 12.10 and virtualbox 4.2.4, installed W7 64-bit. I struggled to install guest additions because I did not get the way it was working but in the end, no issue.
I saved the VM files.
Reinstalled 12.10 and so on from scratch to make sure I could restart my work easily in case of problem. All went well.

Then for long term support and some graphics card driver questions, I decided to go for ubuntu 12.04.
I installed everything again (no error at all) except that now, the VM always aborts when I switch to full screen (or the other mode).
I don't understand why. I had once an error message making my think that there is an issue graphics driver but I'm not sure nor I know how to get a hold of logs.

Does anybody have a clue about that ??? How to move on?

I may try to reinstall 12.10 but for work purpose I would have preferred 12.04 for LTS but I'm not very knowledgeable about Ubuntu/Linux.

Thanx a lot in advance.