2005 Cadillac CTS Clutch Pedal Failure

When coming to a stop (usually in stop and go traffic where the clutch is constantly being used), upon disengaging the clutch, the pedal loses pressure and drops to the floor. If the pedal is lifted up and pumped the system appears to regain pressure but only intermittently. Once the clutch needs to be pushed in again following this it typically reacts the same way. The entire hydraulic system (master & slave cylinders and TO bearing) have been replaced and the problem is still occuring. All fluid levels remain the same when it does occur and once the car rests for a short period of time it usually operates correctly. I have noticed that it almost always occurs in traffic or city-type driving situations, and I have recently noticed that the shifter appears to tighten up and squeak when being moved between gears. I am baffled, as are many others, as to what the issue is? Do you guys happen to know?