Brand new scrambler bike loss of power when the bike shakes on rough terrain! what can that mean?

Question

I have a brand new Brixton BX125 X, the bike has an engine based on the old SUZUKI GN125 but has an EFI to comply with euro 4 and it is equipped with knobby types and light off road details all around like design, geometry etc. I've done about 300km on it on road and yesterday I have finally put it to test on a dirt road with light to medium rough surface. After entering rough terrain and it started shaking the bike started loosing power all at once like the engine power was being cut off! but the engine was still running , the lights were on, etc! awhile on smooth path the bike was spotless but as soon as it started shaking the rev counter would drop immediately and the throttle wouldn't respond anymore, the power would simply go instantly, I then had to go down to 1st gear and as the revs would slow pick up the throttle would engage and the bike would regain power. That happened repeatedly as I had to make my way back home form where I was and the total ride time was about 40mins. It felt like the bike was going on neutral on its own even thought the gear lever wasn't dropping. I stood on the pegs to be sure my foot was not touching the gear lever and it was the same, after shaking the bike bike would stop working! I bought the bike to do light off road close to home and it is advertised for that so what could be causing it? many thanks for any help!