clutch ideas please!!

So this is the problem I have a 51 plate td4 gs. I have owned it for five months. when I first brought it I noticed that the clutch was slipping the dealer I brought it from replaced the clutch and slave cylinder free of charge, great. problem is sometimes the clutch still feels right in the floor. It drives great 98% of the time and then sometimes it feels like the the gear change is really notchy and the clutch isn't engaging properly. thing is when this happens I know that if I leave it overnight the next morning it drives and changes gear like a dream, smooth as a baby's and biting point up from the floor. how can it be fine for ages then feel like the clutch is barely doing it's job. I have checked fluid levels they are fine no loss of fluid at all, bleeding it seems to make a difference for a few days/ weeks but then it happens again. It was terrible tonight on the way home from work but I can guarantee it will be perfect in the morning.. Any ideas please folks..

By the way 5 months and this is so far the only problem, so save yourself the typing time I am not going to BURN IT!!!!!

If the slave and clutch were changed then surley the only thing left is the master cylinder.

Thanks for the reply MJ but would I not be losing fluid from somewhere if the master cylinder was gone? or am I missing something.. Also what would be the cost of replacing the master cylinder?

scored master cylinder may give issues

check where the master cylinder connects to the clutch pedal. Mine used to shift to one side every now and then and make it nearly impossible to change gear without burying your foot into the carpet as this would reduce the throw on the cylinder push rod.
I added a nlyon washer and a circlip to make sure it stayed central and this cured my problem.

master cyl. seals getting spongy perhaps.

Thanks for the advice, think I am going to replace master cylinder to be certain. What would be the cost of replacing the master cylinder by the way?