Ubuntu Server 14.04 64bit on DELL 1950

I use the install CD to prepare my DELL 1950 equipped wit 8GB of memory and two 600GB HD in RAID1 managed by a PERC 5/i RAID controller. The BIOS was 2.7.0 (last available from DELL) and the PERC FW is updated too. All the install process was successful, so i reboot the system. The first time the boot was ok but as soon i reboot again, the system get stuck with these last messages on the console:
...
random: nonblocking pool is initialized
init: plymouth-

upstart-bridge main process (232) terminated with status 1
init: plymouth-upstart-bridge main process ended, respawning
init: unreadahead main process (235) terminated with status 5
Adding 15625212k swap on /dev/sda3. Priority:-1 extents:1 across:15625212k FS

after that i need to power-cycle the system. This problem is not systematic, but 50% reboot get stuck in this way.
Given that i gave a second DELL 1950 in the lab (with the same HW), i repeated all the install process in the same way, and i get the same problem again. I manually partitioning with XFS the disk during the install: 512MB in the boot partition, 30GB in the root partition, 16GB swap partition and roughly 550GB on the var partition (all the remaining left space).

After a complete pass of Memtest86 i get Errors: 0.
Moreover, i install from CD the Ubuntu Server 12.04 64bit and try to reboot the server more than 30 times, without any error.

Can you help me to debug ?