I had the same issue except it was more frequent and worse. Users would have it happen at least 6 times a day and sometimes last up to a minute. I eventually found a solution however.
Using instant clones wasn't working because the way the video memory worked would cause slowness and lagging or to not appear on higher res monitors...
So using linked clones I noticed the freezing would occur if Auto-detect settings was enabled. I switched it to specify settings, Number of displays to 1 and total video memory to 8. Regardless of that the linked clones would extend to 4 displays and go to a higher resolution with better performance (the instant clones would not).
I think upped the CPU's on the parent from 2 to 4 and made sure to make that change in the VM options as well and confirm all 4 CPU's were showing in device manager on the VM.
Added CPU's seen by Device Manager but not by rest of windows 7
After that people stopped seeing the freezing issue. I was so angered by this issue and I came across this and made an account just to post this to hopefully help someone else going through this.
