Server HD Read Write Times

I am truly at a loss and hope that someone can explain this problem to me.  I have a SuperMicro server with 4 AMD Opteron 12 core processors.  It also has 64gb of DDR3 ECC memory.  It is running Windows Server 2012 R2.

I am running hyper v with 3 virtual 2012 servers and 1 linux proxy gateway.  I also typically have a Windows 7 VM and a Windows XP VM running on this host.  I have had 2 or 3 other Windows vms running at various times with no problem.

Just this morning I had an issue with the Linux gateway not running correctly.  Eventually (with some help) I found out that the HD access speeds were dropping from 500-600 mb/s range to 10-30 mb/s range.  I discovered this using DD command from the Linux command line.  I began testing the HD speeds on the host using CrystalDiskMark and discovered that I was only getting somewhere in the range of 40-120 mb/s on the physical Hard Drives from the host OS.  Keep in mind that I am using standard Sata drives 7200rpm.

What I don't understand is how I could be achieving better speeds on the VM than the host OS can get.

Also, I don't understand what could be causing the rate drop in the VM.  The one thing I discovered is that the Linux VM seemed to hold its higher speeds until one of the Windows Server VMs was turned on.

I also purchased a new Sata drive and installed it and discovered that I was getting about 50-75% better transfer rates on that using CrystalDiskMark from the host.  I honestly expected a much bigger increase than that.

After the single Windows server VM was completely started up I reran the tests on the host drive that contains both VMs and I got a range of 90-120 mb/s.  I then ran a test on the Linux VHD using the DD command and it was now back up in the 400-500 mb/s range.  I also ran the same CrystalDiskMark test from inside the Windows Server VM and was getting 130-150 mb/s range.

I'm just looking for some kind of explanation for what might be going on here.  I have been running VMs on these types of HDs for years and have never had these kinds of issues.  Could there be something else going on here?