Horrible Lag in Flight Simulator FS9

I've been on PROPS Racing at WestCoastATC for close to a year now with no problems.
This is online flying of P-51 Mustangs where 10-15 pilots follow a pace plane over a course
and then we dive to the track to race around several pylons.

For some reason, for the past 3 weeks, in every session I now have this horrible lag.
The game itself never lags, just the other players.

When following the pace plan as well as observing other players, the plans appear and
disappear, then reappear in different places as though I pause for a second at a time.
I've always enjoyed a flawless game except for one fellow who is on dial-up and he shows
lag to the rest of us all the time.

It doesn't effect my frame rates, which if I set to unlimited I get
about 150 fps. My connection speed is 3,000 kbps. My upload is about 300 kbps.

AMD Athlon 64 Processor 3200+ 2.2 GHz 1024MB Ram
Radeon 9800 Pro

I switched cable modems, same thing. I switched Ethernet cards, same thing. I
uninstalled FS9 and reinstalled in new directory-new location
on my hard drive. Checked for spyware with several different
programs, checked for viruses with several different programs.
Clean as a whistle. Defragged, cleaned up and polished everything
I can think of. Same thing, horrible lag.

How can this be? Horrible lag on a very fast computer with a great
connection? It's not the server because everyone else can see everyone else just fine
with no lag. We play on a dedicated server. I can be the only one in the server and follow
the pace plane for awhile and then I start seeing lag.

My computer temps are all normal with good airflow.

I'm wondering, since FS9 is so CPU intensive, if perhaps my CPU is going bad?

Any ideas?