Did you replace the pilot bearing(if it uses one?). It's usually pressed into the flywheel or crankshaft. If it' worn out it'll cause the exact symptoms you explained. I had one pile up in my truck and it took a day or two to figure it out. When I would sit stationary with the clutch pressed while in gear it would try to take off a little bit like I was letting the clutch out. The needle bearing in the pilot bearing were piling up and grabbing the input shaft. I'm terrible at explaining but hope that makes sense.
The only other thing I could think of is if the clutch linkage isn't adjusted properly? Do you have a hydraulic or cable clutch?