Weird render-loop while using Cycles scene for compositing

Since I’m still not that familiar with Cycles I’m trying a slow transition. That means that I still work a lot with the “old” render engine, since I can produce my desired results more quickly there.
I created a Scene with a cogwheel in the center, some planes with textures in the Background and an overlay for additional information regarding some parts of the cogwheel. For easy compositing I’ve split the scene up in four different render-layers, namely “background”, “cogwheel”, “foreground” and “text_overlay”.
Since I wanted some Sparks flying through the scene from the right, I absolutely fell in love with Cycles’ particle info node! I used this node to divide the age of those particles by their lifetime and use that as input for a color ramp, which will influence the sparks color. In short: Those particles will fade from bright yellow to red and black over their lifetime.
Since I created the cogwheel and the rest of the scene with the old render engine, I created a new scene for those sparks, using Cycles.
Now in the compositor I set up all the nodes for the scene and then wanted to mix in the sparks with a color node using “add”. Everything looked the way I wanted, but here’s the problem:
The scene rendered as usual and then switched to the Cycles scene. It rendered the current frame as intended but after finishing it, it rendered the same frame again! And again. And again. And so on.
This way I couldn’t render the animation at all, since it got stuck on the first frame rendering the Cycles scene 'till the end of days.

I made a small work-around by rendering the whole sparks animation as a .avi video, and used that for compositing.
Now my question is, is this weird render-loop a known issue, did I do something wrong, or is this not possible yet?

Here’s a shot of the scene:

I will upload my compositing setup on demand, if needed.
Thanks in advance for your help!