When you replaced the trans, did you use a jack for support? Or just do a bench press of the trans? In the process of getting everything lined up if you let the trans hang in the splines while you rest your arms before the pilot snout is engaged with the bearing, the force can bend the clutch disc. The result is usually clutch drag as you describe or clutch chatter on take off or both. If it doesn't chatter you may be able to rectify the symptom with an adjustment. But you can only go so far until the throw-out bearing is always engaged. In that case, the bearing will eventually fail and/or the clutch will slip.
There is also the possibility that the rivets, on which the diafram spring pivots, are loose and moving slightly or the spring is distorted. I had this happen on a reman clutch a few years ago. It was replaced on warranty but I still had to do the job twice.
