Since you're running XP you could try running FS in a window and resize it so you can run Task Manager along side it. If you select the Processes tab on Task Manager you can watch cpu and memory usage while you fly. If some other process is interfering you might see it.
The F16 I mentioned affected fps by using a lot of cpu. Weather can do that too. But so can a lot of disk activity. I have noticed a drop in fps when fs9 is doing a lot of disk work. Reading in new scenery, for example. Do you hear the disk rattling around when the fps drops off?
I guess you could try a process of ellimination. First, disconnect your PC from the network, shutdown your virus checker, firewall and anything else you can do without for a while. Try a flight and see what happens.
And when was the last time you defragged your hard drive? If files are scattered all over the place then Windows is going work harder to find them.
You say it happens evey 5-10 minutes. Are you flying with real-world weather over a dial-up line? FS updates it's weather every 15 minutes.
As you can see there are probably a lot of possibilities and it might be a combination of things.
gw