If you are using an aftermarket pitts muffler with large exhaust outlets the muffler pressure is low and it makes the needle settings creep a lot. There are two fixes. I'm running a Perry regulating pump using the bypass system. ("T" fitting at the carb returning excess fuel to the tank) You can also restrict the muffler outlets some to increase pressure.
Also, be damn sure you aren't getting air leaks at the carb. The mds retaining bolt is very close to the crank and has very little sealing area. The new C2 carbs have a longer throat that engages the casting near the crank. I was told that the lower O-ring was not needed with this new design, just the on at the top of the neck. Not true! It would air leak around the carb retaining bolt when the engine heated up and the crankcase expands. I would group most "runs great on ground, dies in air" to this leak.
I'm running a lower and upper o-ring. Some use sensor safe RTV to accomplish the same.
Its a hard leak to diagnose, the way the carb and crankcase heat and cool it rarely shows up on the ground. I only found the retaining bolt leak while doing some perfomance testing on a test stand and then ONLY after a particularly hot run.

If you seal up the carb, run low nitro fuel, and get good muffler pressure or a pump, the MDS runs great.