To begin with, when having the car stopped for anything more than a second or two, you should slip the trans in neutral and release the clutch. Once the light turns green simply, kick in the clutch, throw it in neutral and take off. This will save an ENORMOUS amount of wear on your throwout bearing and is also much easier on your hydraulic system.

As far as your problem, it is most likely hydraulics bleeding off pressure somewhere. Pull the floormat and inspect the floor and cylinder itself for leaking oil. If it is leaking at the master cylinder get a rebuild kit and a cylinder hone. Make sure you get ALL the pits out during the rebuild. If you don't find anything wrong at the m/c then check the slave. Slip it out carefully with lots of light available and see if it is wet in the cylinder past the piston. If so, rebuild it the same way.

Good luck,