Remote desktops and such

Hey guys

I am at my wits end on a problem that I am having with Remote desktops and such.

Here is the situation.

I have two Hyper-V servers, each with a mix of WinXP and Win7 desktops.  The Win7 machines were all created from scratch and have absolutely no problem.  The WinXP machines were created using Microsofts SCVMM 2008 R2.  The creation process worked better than I had dreamed and nearly everyone's desktop is now on the servers.  When I created them I left them without a NIC until I had logged into them thru the console.  I then took the Virtual machines off the domain, changed their name and and put them back on the domain.  
The people seem to have no problem logging into their remote sessions from inside the domain but from the outside they have intermittent issues.
Ok, I think I know what the problem is.  Could their MAC address be causing this issue?  Do I need to go in there and make sure that all of the machines that were created using the SCVMM be on Dynamic MAC addresses?
The firewall uses Port redirection so I would make each one have a different Port at the end of the IP address they would use to get to their machine remotely.  I.E.  my.domain.com:9999  my.domain.com:9998 etc  
Right now I am looking at the network configuration for them and it says the MAC is static.  Should I make it dynamic?
Any help would be much appreciated!!!