Remote Desktop Server slowdown/pause issue

All,

We have 10 thin clients running RDP to a virtual server, RDS. RDS is
running Windows Server 2008 R2 Standard x64, has 10gb memory, and the C:
drive is 255gb, using 34gb. The thin clients are XP embedded from 10ZiG.
RDS has no domain controller responsibilities, whatsoever. It's only
function in life is to serve the thin clients.

Every so often the thin client users experience a slowdown because the
server, RDS, is busy doing . . . something. What that "something" is, is
the unknown. The thin client sessions, for all practical purposes, come to
a complete stop. Then, 5-10 minutes later, everything is fine and away we
go. It typically happens 2 or 3 times a week. No discernible pattern.
Might happen 3 days in a row, then not again for a week. Seems to be more
often in the morning as opposed to the afternoon. If I look at Hyper-V at
the time it's happening, Hyper-V says the CPU utilization is 10-12%, which
doesn't seem excessive to me. But I can never get logged on because of the
slowdown. A frustrating issue for all involved.

Each of the thin clients will be running 1 or 2 System i sessions and a
browser (either Chrome or Firefox) as we use Google Apps for business.
Individual thin clients _might_ have Excel, Word, Publisher, stuff like
that open.

Since I am a one-person shop responsible more on the business side than the
tech side these days, we have a Managed Services contract with a company to
handle these sorts of things. For whatever reason, they cannot figure out
the issue. The only idea they have had so far is Group Policy update, but
have since ruled that out.

Nor do they seem to have resources available to them to resolve issues when
they can't. (That lack of resources on their part is something I will have
to deal with in the near future.) Their next step is to shotgun it by
analyzing everything in sight at my expense. They want to involve MS, and
the thin client manufacturer, and do some network routing analysis. They
expect all this to cost me 20-30 hours of billable support. And have
already said if this doesn't resolve it, they'll look at other things. You
can imagine my joy at this.

Does anybody have any ideas on what I might look at as being the cause of
the slowdown? Both the support company and I have looked at the Windows
logs to no avail.

Thanks.