I am getting this issue as well, and I know how to replicate it exactly, it seems that as soon as your rear wheels leave the surface for any amount of time and then touch again, you get this boost. I can replicate it most when travelling at around 70MPH (110KPH), any steep gradient such as Highway ramps, bridges (Especially the on the C-D road just before the Duisburg turn), and you can get it to happen pretty much any time you go across the rail-way rails at a crossing. This bug also often makes you lose control, especially with the updated physics. To note:
G29 + H-Shifter + Clutch (Real Manual)
When it happens
At any steep-ish gradient in the road
Turning corners too tightly at low speeds (Especially out of the Mechanic)
Going off-road and hitting a bump in the terrain
Being hit hard enough that your car is in the air for any amount of time and then hitting the ground again.
Video Footage from going up a Highway ramp:
Goes from 70MPH -> 85MPH instantly, this was when it first happened to me.
Video Footage from going over the C-D road (Duisburg side) bridge, here I could replicate it most by travelling at exactly 70MPH (110KPH):
Goes from 93MPH (limit) -> 104MPH - This also makes me lose control, as you cannot regain control with the new physics if you are travelling at over 93MPH, due to the speed bug.
Edit: Forgot to mention, for this to work you need to have 100% throttle, I was trying it with less and couldn't get it to work.