How do I Fix Pixel Jumping

I use a bullet speed 100. A sprite move to left. I notice it is often speed up a bit before back to normal. How to fix this?

I dont understand, Explain better. upload a file, or upload a image.

[attachment=1:1m9ll822][/attachment:1m9ll822]

This is a snake. The head leaves trails, recorded in an array. The body picks them up.

Often when the FPS goes down, the head seems jumpy. Leaving its bodies behind.

[attachment=0:1m9ll822][/attachment:1m9ll822]

The lower the FPS, the farther the distance.

When the FPS goes back to around 59. The snake head moves steady and so the bodies.

To move forward I use bullet behavior. Speed set at 100.

https://www.dropbox.com/s/5v8nrpfkfbvqpr0/snake3.capx?dl=0