You may find that the problem is not with the clutch but related to the solonoid. this is a relay that has a low current to switch it on and heavy duty high current contacts that are in series between your battery and the starter motor. if these contacts become burnt or dirty then the high resistance prevents the starter motor from turning over the engine reliably. the easiest solution is to replace this unit. It is easy to find on most mowers, just follow the heavy duty cable from the positive terminal of the battery. the solonoid will have this cable bolted to one side, and a similar cable bolted to the other which goes to the starter motor on the engine. There will also be two smaller spade type terminals which are the coil connections (low current) these come from your ignition switch. Also check that these terminals are clean.
Bill
February 2009