If your tranny is slipping try holding the cheeks tighter and/or useing a little less lube.
The only part in your G/box than can slip is the clutch. If a gear is missing enough teeth to slip then the car would not even be moving.
How sure are you the clutch was replaced? and who replaced it?
Its possible it was incorectly adjusted.
Theres an adjuster knob on the cable just above the clutch lever arm. (sticks out the frount of the gearbox) There should be a small amount of slack in the cable, and the lever should have a few frations of an inch of free play. If the cable is applying any pressure to the lever arm then its exactly the same as if you were riding the clutch.
Turn the knob to lossen the cable a little. (dont pull it, trannys enjoy that)