I here ya, however, if you’re interpolation is non-linear then you’re going to end up with frame rate dependent results. You just might not notice it until you hit extremes like 15 fps. I’m fairly confident that your original setup is correct, we just need to address this non-linear interpolation.
Let’s try this. There is a VInterToConstant node, which appears to be linear interpolation. It should be a drop-in replacement for VInterTo. Give that a shot and see if the results are different.