I agree. I have about 400mi on my Grubee 48 now. It seems very temperamental about moisture content changes moreso than temperature changes.
If you make short trips, try starting by just priming 2x and pedaling, instead of choking. This seems to help me get more consistent performance out of it.
As far as speed changes, mine goes about 32 on flat ground with no wind, and bogs down a little to about 27 with a headwind, maybe 25 with a real stiff headwind. On wet, foggy days, it seems to run "richer", more prone to make the "4stroke" sound at half throttle. I don't worry about it too much. I don't want to move that stinking clip for every weather change. Just make sure its getting enough fuel and oil. Tune it for the weather you get the most.