I seem to be having an issue with my HF1000 I received a few weeks ago. About 1 in 4 times (maybe a bit more) using the throttle more so than pedaling it judders, the last time it happened it was very bad, I was using it from a stop. Kind of like a car stalling or stuttering or skipping. Sometimes it feels like I'm on a cobblestone road but it's flat pavement, that will happen with I pedal too. I've only put about 25 miles on it. Any knowledge would be appreciated.