The problem is you have a max speed of 600, but only a acceleration of 125 and declaration of 75. If you now are going to the left and hit the left boarder with full speed and now you want to the right. The speed has to decrease from 600 to 0 and this takes its time, because of your acceleration before you can go to the right. The same is for right and the top. The bottom is a little different. You are using the Bullet-Behavior for gravity, but the gravity increase over the time, even if you sit on the bottom. So if you wait a little the gravity is over the max speed and you can not move anymore. You can see that if you go in Debug-Mode and look on the gravity of the Bullet-Behavior.
You shouldn't mix so many moment behaviors. If you want use the physic-behavior you should use only the physic behavior, and the physic-behavior doesn't like the solid-behavior. So you have to be cautious what behaviors do you use.