GRUB problem with VMs after Proxmox reboot

Hi,

We have encountered problem with GRUB while VM is booting up. The problem has totally random behaviour.
Error:

Press F12 for boot menu.

Booting from Hard Disk ...
Grub _

It happens after rebooting Proxmox VE and while VMs are booting up (just after reboot Proxmox VE). But as I said, it does not happen every time.

Our systems:

Proxmox:
Proxmox Virtual Enviroment Version 2.1-1

user@hostname:~# more /etc/debian_version
6.0.5
user@hostname:~# uname -a
Linux hostname 2.6.32-12-pve #1 SMP Tue May 15 06:02:20 CEST 2012 x86_64 GNU/Linux

VM:

[user@hostname bin]# more /etc/*release
Red Hat Enterprise Linux Server release 5.8 (Tikanga)
[user@hostname bin]# uname -a
Linux hostname 2.6.18-308.el5 #1 SMP Fri Jan 27 17:17:51 EST 2012 x86_64 x86_64 x86_64 GNU/Linux


Any hint, suggestion will be highly appreciated.

Thanks in advance.