FPS deterirate over time in MP Flights

[Stratocruiser]

Using P3D v5.1 with frame rates locked at 30, and the program Swift to fly on line with my VA. If I am alone and logged in onto SWIFT I can fly for hours and hours. and frame rates stay pretty much at 28 to 30.
Flying on line with 10 to 12 VA members, all using SWIFT, after about an hour I start to see my FPS degrading down into the mid teens, and after two hours they are down into the single digits. This of course makes it impossible to fly.
We have run several tests and the only thing we can think of is that the Cache (not sure of the spelling) section of the computer is not releasing its contents as we fly, ending up with an overload situation after a couple of hours. However, this only happens when flying on line using the Swift program AND with other players also signed on and we can see each others planes when in range. It does not happen if it is just me on line, or just me flying off line.
Does anyone know where the Cache file is located and can I find a way to make it automatically empty during flight?
Thanks

[Martyson]

Using P3D v5.1 with frame rates locked at 30, and the program Swift to fly on line with my VA. If I am alone and logged in onto SWIFT I can fly for hours and hours. and frame rates stay pretty much at 28 to 30.
Flying on line with 10 to 12 VA members, all using SWIFT, after about an hour I start to see my FPS degrading down into the mid teens, and after two hours they are down into the single digits. This of course makes it impossible to fly.
We have run several tests and the only thing we can think of is that the Cache (not sure of the spelling) section of the computer is not releasing its contents as we fly, ending up with an overload situation after a couple of hours. However, this only happens when flying on line using the Swift program AND with other players also signed on and we can see each others planes when in range. It does not happen if it is just me on line, or just me flying off line.
Does anyone know where the Cache file is located and can I find a way to make it automatically empty during flight?
Thanks

@ Stratocruiser ,

I have used Vpilot and no problems.

Have you asked Vatsim about your problem?