Off road I'm sure you are using the clutch _much_ more than on road, yes ?

OK, brake and clutch fluid is hygroscopic, it absorbs water. When you compress the clutch fluid, it generates heat. Since you are using the clutch much more, then much more heat is being generated. When it gets hot enough, it boils the absorbed water out of the fluid, creating bubbles in the lines. These bubbles are what's messin' witch ya I bet. Out with the old fluid, and in with the new, from a NEW bottle, not one that's been sitting on the shelf in the garage for a year and a half.