Increasing the sim rate will always create a bouncy flight. That is because the amount of normal up-and-down movement is compressed into the accelerated time.
At 4x, the amount of movement that is normally experienced in one minute will be experienced in just 15 seconds.
At 8x, the amount of movement that is normally experienced in one minute will be experienced in less than 8 seconds.
Thus, what appears to be gentle up-and-down at 1x suddenly becomes a wild ride. This is made even more dramatic by the fact each time you bump the rate up the increase is not linear (2x > 4X > 6x … ), but is an exponential increase (2x > 4X > 8x > 16x … ).