So here is the situation, I was using Ubuntu 14.04 32 bits. Removed everything from the partition I had and created an additional Swap partition and installed Xubuntu 14.04 64 bits. When booting I can choose Ubuntu from Grub (I also have windows). Then black screen and few seconds later my laptop reboots. With some black rituals I eventually got my laptop to boot sometimes (hitting it, opening the cd drive at boot, some superstitious stuff). But sometimes it just won't boot and keeps rebooting.
Windows boots just fine
I removed one of the rams (I have 2x2Gb) and suddenly it worked. I tested with many boots and it was fine. I switched the rams and it still worked. So the problem seems to be with the 4Gb size of the RAM. My Swap partition is slightly smaller than the size of the ram, could that be a problem?
Otherwise what can I do exactly to start troubleshooting this problem. I have no idea where to start and what debug messages to read.