I have the same thing going on with the clutch in my 360. I've had it on the bench for a while now but I haven't gotten into it yet. The 500 motor I have in the bike currently has the same type of clutch and primary drive but does not exhibit the same issue.

Something is worn, either a bearing or a thrust washer. If I identify it I'll follow up.

It is quite irritating to deal with, and my 360 did it for 11k miles.

Turbodan,
Thanks for the added information. I was able to ride this one for a mile or so before it was torn down and did not have the issue then. I am sure that it is a thrust washer that is missing and was not replaced by my helper. I just have not had time to inspect every piece yet either.

Hi, I just rebuilt the motor on my DT400B and have exactly the same issue with the clutch under load. Please let me know if you managed to find the issue
 

I have torn mine apart again and found that the end-play in the mainshaft holding the clutch pack had excessive movement. While all the original shims went back in this, I have found there needed to be almost .050" shims needed. Why? I have no idea since the listing for these shows .3mm and .5mm for shims, and nothing close to the 1.25mm I needed. Even adding that much, I still have some end-play and need to add more due to the clutch still slipping some under load and the cable free-play disappearing.
My only assumption is that the replacement KOYO bearings were a tad thinner than the OEM.