There are too many things that can cause the sputtering, my P-51 did that with an inverted OS 91 in a power dive. I added a $28 Perry Oscillating pump and it seemed to take care of it, however, it could be a lot of things in your case. Try replacing all of the lines and make sure your clunk isn't stuck somewhere in the tank. Another thing is you might consider is a bigger prop. When the engine overrevs, you could be floating a valve causing the engine to sag and surge in the dive. I recommend you try a 14x6 or 13x8 with the OS 91. The 14x6 Master Airscrew looks nice and scale like on the warbirds as well as howls at high rpm which adds to the excitement. It won't go very fast but it will sound fast. A 13x8 Master Airscrew Scimitar will make you go fast.

Good luck.