sounds like clutch may be getting worn. sometimes if you shift too softly, you can slip out of gear and grind the gears a little. the more that happens, the easier it is for it so slip out.

 

Take it out, and you know how when you shift you pop in the clutch, and roll off slightly at the same time to click into the next gear. Well next time you pop in the clutch and don't roll off the throttle and just shift, then just release the clutch and if it is starting to sliip you'll hear it. Rpm's will drop to the proper RPM depending on gear and speed, but if clutch is going, you'll have slippage and the RPM's may drop slightly but not too much, or may just take an long time to reach the proper RPM's. That is a stress test for your clutch, but it can overheat it, so don't do it too much! If you are passing that test, then it might be something internally wrong! Like rounded dogs on the gear and it is popping out when under a load. For that you need to drop the motor split the bottom end and replace a few gears. Not hard, more annoying! Besides i hate having to deal with wiring and hose routines when i put a motor back in! So hope this helps