Hi, I had the same prob. kinda what happen with me was that I had toghten the throttle side to tight and cracked the threads & it stayed on until the next time when I checked to insure they were both secure and it fell off in my hand. I did do a temp fix with gorilla glue, I glued the threads back on without the end cap & when that was dry I put the cap back on just tight enoupf so it wouldn't loosen up. Then I ordered a new throttle. It cost about a $100.00 for a new one you get the grip and stock grip end all in one. I replaced the broken one then put the new end cap back on but this time I did not crank on it just gentle hand tight until I felt resistance then stop. Have had no problem since.
Good luck!!
Good luck!!