This would be because when you're in the air, your velocity for keypress is greatly reduced. (Meaning when you're on the ground you can move fast enough when holding down a direction key to offset your reduction code. However, when you're in the air, this is cut almost to zero.)
You'll have to figure out a way of just simply limiting it so that it doesn't exceed a certain figure. i.e. figure out the player's maximum normal velocity, then apply your weight stuff, and finally lock the x and y self.velocity to not exceed the x and y of your limiting figure.
This code isn't happening every frame is it (e.g. in PlayerPreThink or PlayerPostThink or one of the functions they call?). Because if so, it's also framerate dependant. If you have a slower computer, you will move faster (fewer reductions per second), a faster computer and you will move slower (more reductions per second).
I suggest doing what Dr. Shadowb0rg recommended in his last reply.
, except for when I jump.