If you use fps min, I think, you're limiting the minimum FPS to 50, but only for calculations. Pentium II cant give you 50 fps...
Another thought to try out,
think how many frames you have for this certain animation, and how fast you go through them in frames/sec and model frames/sec.
If you have to, reduce or increase the number of frames on the model itself (will need additional animation
but its the only other thing to try out after everything else...)