Is it normal for a wind of 8kn (described as "gentle breeze") to toss the plane around like a feather in a tornado?

At least on two separate occasions these "gentle breezes" have been causing insane turbulence with the planes bouncing several meters up and down without a break after getting a bit of altitude. The on-board speedometer is swinging wildly between 60 and 50 knots and the stall warning is constantly appearing. For the extremely stable B.E.2 that I'm currently flying this is a mild inconvenience but for Fokker E.III it meant I couldn't let go of the stick to wipe the sweat off my eyes without instantly rolling 90 degrees and/or entering a spin.

What is the strongest possible wind that won't cause the flights to be cancelled?