I deleted the file and restarted a new session...same results.
Here's where it gets interesting. I loaded up the TN version and performed a takeoff, just for a sanity check, and all was perfect with the RPM's. Instead of shutting down XP and restarting in the NA version, I just immediately loaded the NA version and executed a takeoff. No bouncing of the RPM's. It was smooth as silk. Next, I shut down XP and loaded the NA up and the RPM's were unstable again.
Does that help in any way explain what the issue could be? It is repeatable as I did the above twice and had the same results.
Thanks @Coop for being patient with me and this problem.
<edit> To be clear about all this, all I have to do is bootup XP with the TN version. I don't have to turn on anything (battery, etc). Then, immediately load up the NA version, and everything works as expected. But, if I boot up XP and start the NA version straight away, I'll get fluctuating RPMs.
Scott