Riding mower transmission slipping?

My mower has about 8 settings for speed and uses the lever positioned in the notches to change speeds. The first 3 or 4 settings (low gears) don't do anything when I release the clutch, but the 4th or 5th will lurch in and out. If i put it in the 5th or higher it works fine. Is this a linkage problem, transmission fluid problem? What can I do to troubleshoot and fix?