Sounds like your low speed needle it a little rich which is causing the engine to bog down a bit until it burns off some of the extra fuel when transitioning to the high end. When you adjust your low end needle only adjust it about 1/8 turn at a time. You don't want make quarter or half turn changes like you might on the high end needle - this would be too much and throw things off worse than they are now.

If you are flying in the cold you do need to run a little richer than normal since the cold affects the timing of two stroke engines.