The problem:
The speed of an object with Platform behavior varies wildly from tick to tick, sometimes as much as 50px per second or more. This effect is most noticeable when the Platform behavior has a low ground and air speed setting.
How to recreate:
Download and run the attached .cap file. There is a text object that shows the speed of the player as you move back and forth. If you watch closely you can also see the object speeding up and slowing down. It even goes over the max speed set in the properties.
What should happen:
After the initial acceleration, and while the controls continue to be held down, the platform object should maintain the max speed setting at a constant rate.
Platform speed bug example