PowerEdge R210 II Windows Server 2012 R2 Unresponsive

Hello. We have a (remote leased)  PowerEdgeR210 II for about two months. From the beginning the server, running Windows Server 2012 R2, sometimes become unresponsive; the server is reachable by IDRAC, some services keep on running (like the mail server) but IIS and remote desktop become unreachable (published websites not available and remote desktop stacks on "configuring remote session"). If this happens while I'm remotely logged in I can keep on working on the server but UI becomes unresponsive too (cannot open new software or change active windows or open task manager or event viewer); anyway RDP connection remains alive. The only available choice is to power ON and then OFF the server with IDRAC. When the server reboot no events are visible in the event viewer: the event viewer seems to stop at the time when the server became unresponsive and start again to log with the kernel power ON event.
It happens randomly: sometime server is up without problem for 5 days and then becomes unresponsive, sometime it happens 2 times in sequence (in a short period of time like 2 or 3 hours). Last time it happened (it was the first time I could see something in the event viewer) a lot of "ID 129 warning Reset to device, \Device\RaidPort0", was issued befor the event.
On the server are running a mail server (hmailserver), IIS and nothing else.
I updated all the drivers (AHCI controller too) and bios to the last available on Dell download area. Available RAM is 16GB and usually is used around no more than 30%,
Does anyone have an idea to sort out the problem or have exprimented similar issues?
Thank you.