I had similar symptoms after putting in my first dual disc clutch. When I wrote about it there were a lot of suggestions - dragging pilot bearing, release mechanism not fully releasing, etc.
When the clutch started slipping and I replaced it with a new dual disc last summer, I also has the transmission rebuilt. After that, and after a short break in, the shifting problem went away. Not sure whether it was the new clutch or the rebuild.
For what its worth, while WDaniels is correct that the stock clutch release system is not adjustable, an adjustable one can be purchased from SouthBend Clutch. It's heavier duty (larger slave cylinder) and there's an adjustment on the rod that connects to the clutch pedal. Surprisingly, they are about the same price as a stock one from the dealer.
-Jay