clutch problem

hey there guys. I have a 95 gst and im having some clutch issues with it. I recently replaced the trans, master cylinder and rebuilt the slave cylinder and also replaced 1 of the lines. The problem is that ever 25 - 50 miles the clutch starts to stiffen up and then gets real stiff and starts to slip. I just lay under the car open the bleeder let it spray then close it and it is fine for a little but then does it again. The longer it goes the worse it gets,after so long it wont even push in enough to get it in gear. Any help would be appreciated.

the clutch gets stiffer??? hmmmmm.......... let me think