Resistance issue and power increases

Im having resistance issues whilst in zwift race. When im racing, my power suddenly increases or the resistance gets really hard with no effort in my pedalling stroke. This causes me to suddenly sprint out of the group in a race. Im using a apple tablet, and i switch to my iphone when this happens, but the problem exists on both devices. I have a Kickr 18 smart trainer. I had this issue before but then updated the software and worked…but now i updated softwarre, did spindowns and still not working.
Any solution for me please is appreciated.
Thanks