I think your issue could be one of two things.
1) The float in the carburetor may be fuel saturated. If it is leaking and not closing the needle valve all the way, it could be causing the carb to flood slightly, which would make it slow down a bit when changing the moving vector of the lawn mower.
2) The needle valve itself is not seating correctly and letting too much fuel through when you don't want it. This is less likely, but could be happening.
Even if either of these two things aren't in play, I still believe its a fueling issue. I would bet either a carb rebuild would fix it (if you can get the rebuild to work correctly) or a new carb would definitely do it. While a new carb would be more expensive, I have been opting for this over a rebuild kit as I can never seem to get the rebuilt carb to function correctly. Getting a new carb for your small engine will save you a lot of time and hassle in the long run over the rebuild as well. A new carb will usually cost about 3 times as much as a rebuild kit, but is well worth it in the end, in my humble opinion (I always consider: What is my time worth?).