Proxmox freezes randomly

Hi,

For a coupel of days mine proxmox servers stops responding at random times i need to hard restart the server to get it back running.

I noticed this happens when one VM has a lot of load on it or when i start Firefox on a Ubuntu VM.

There is nothing in the logs that gives an explination and the VM settings are whitin the limits of the server.

Any help?

Regards,
Gamerh