Only things that could be causing this is your oil, a badly slipping clutch, or worn clutch springs.
First off, what oil are you using? Brand, weight, and does it have the energy conserving logo on the back of the bottle?
Since you already have your clutch out, measure the springs. In the manual is a spec for the springs, if they fall out of this range, you need new ones.
Finally, when you put it all together, make sure your cable slides freely and make sure the clutch is properly adjusted.
Most of my money is on the oil though.
The ride IS the adventure. The destination is just to get gas!