Interesting. The first time I observed this behavior was on a hot day (about 85F/29.5c). The throttle seems to work independently of pedaling... i.e I don't have to pedal with the throttle but if I do pedal, I feel the bike accelerating until the pedals spin out (I run out of gears and am going fast enough where I cant apply much force on the pedals). And while pedaling I can modulate the throttle and feel the difference between 50% and 100%. However, after this problem comes up (after 5-8 mins) then the throttle has little effect.There is a thermal sensor buried in the windings, and it IS designed to lower the power to the motor when it reaches a certain temperature.
To be of much more help on this, it would be important to know what you were using for a throttle setting during those 5 to 8 minutes preceding the slow down.
2 more points. 1st, you can easily monitor the load on the motor by simply keeping an eye on the watt meter. It's not hard to imagine the internal heat build up with a constant over 1000w draw for instance.....
2nd, on some bikes, if the throttle is engaged while you are pedaling, the throttle is more of an on/off switch than a throttle. To get the throttle actually modulating power/working as designed, you have to stop pedaling for a second, then engage the throttle.
But one thing that I just noticed is that the watt meter on the display only shows power from pedaling not throttle alone?
I'll try to observe the wattage on the display on the next ride. (Don't think it will be possible to get a picture of that due to the speeds at high wattage).
But bottom line, you believe that the observed behavior is normal in hot or warm conditions?