Start with the less expensive things first.. have your seals, fluids & reservoirs checked to insure there is no debris nor lack in fluid pressure for clutch operation viva proper shifting. Also check out the lines to the tranny, and make sure lines has free flow to tranny's clutch pack.
Also "listen" intently to any churning nose coming from tranny while in or out of gear.. The throw-out bearing normally will go first before clutch plate.. when it does, you'll hear it...
if it's quiet as a mouse..then the problem could be within the clutch..however, when the clutch is going out...you'll notice the clutch slipping when going up a steep hill in a higher gear.. also a scraping or grinding trying to put it into gear...
Since you did not mention any of these problems, leads me to believe that the problem is possibly within your clutch fluid and/or line to tranny.
A trustworthy trans. shop should be able check this out for you too and evaluate the problem..
**Major repairs are most often the cause from long term poor maintenance**