Sometimes it is necessary to change the tension on the trem springs, accessible thru the back plate of the guitar. I've had a couple Jems that I bought which gave me terrible problems staying in tune until I had the springs adjusted. On the other hand, when you finally do get the bridge adjusted correctly, it will never go out of tune unless you drop the guitar. Of my trustier Jems, I have a couple that I can literally leave unplayed for a year or more, then bust out of the case and start playing immediately, still in perfect tune and intonation. The Jems (and UVs) are the only guitars I've ever owned that I can leave unattended like that and still be able to count on for immediate service.
I think there are some tips in the Tech Setup section here at Jemsite for adjusting spring tension. Myself, I am a mechanical moron, so I just take my guitars to a shop and have them professionally set up, but I have plenty of friends that find it no problem to adjust and maintain these type of trems themselves. Good luck!