Small Engines (Lawn Mowers, etc.): Toro Lawnmower drive gear slips, transmission gear box, good shape

Question
I have a Toro lawnmower model # 20676. The mower starts and runs fine.  It is has 3 drive gears.  The problem I am experiencing is when I engaged the drive gear in to second gear and operate.   It engages and runs fine for a few minutes then intermittently slips and does this continuously at random .  However, It does NOT do this when engaged in first gear.....operates fine. I have not tried 3rd gear. What is causing this and what part do i need to replace to correct this problem?  Any help is appreciated.  Thanks!

Answer
If the drive belt is in good shape and you have no problems in first or third I would suspect the transmission gear box.  However, you need to very the external parts are not the problem...I have had those little wheel spring cause issues.

Did you look up your manual at this site:

https://homeownersolutions.toro.com/portal/server.pt/community/product_center/225#manualsResultPage

Eric