Changing Frame Rates and Artifacts with Optical Flow

I'm shooting at 50fps with an Atmos Ninja V (camera is A7s ii). 

When I import to Premier I use Interpret Footage / Assume This Frame Rate / 25fps. 

So fine. I've got my footage half speed as it should be. 

Often its a bit too slow and I want to increase the speed to 150% (or 75% of the speed at which it was shot).

I'm using the Speed/Duration function, but when I pick a time interpolation option, Frame Sampling is jerky, Frame Interpolation makes me feel drunk and Optical flow creates weird artifiacts. Image attached. 

Is the problem that I am trying to choose a frame rate in between 25 and 50, which would be 37.5? Does it have to be a multiple of 25?

Any help much appreciated on how i can get smooth footage at 3/4 speed. The Atmos doesnt have a function to shoot at 35fps, which i appreciate would solve my problem.