Hey Lori,

Can't recall ever having trouble with it - that bike probably hasn't been rode in nearly two years - which could be part of the problem.  Here are a few things to try.

First, and I have to do this on both of my tri bikes...if I am grinding up a hill, and the pedaling is pretty tough and there is a lot of tension between the pedals and the chain, I have to ease up my peddling and then switch gears in the front.  Otherwise, the tension will often times make the chain pop off.  That is fairly common.  A good example would be pedaling up a hill and its really difficult and you're pushing hard...and in the middle of that you switch gears while still pushing hard into the pedals.  That's when the chain will often pop off.

Secondly, when you change gears, either front or back, if you don't push the handle all the way over, it won't engage the derailleur fully, therefore making the chain not hook completely onto the ring.  It will then slip off.  This is common as well.

Third, if you change gears with the chain offset between the front rings and the back cogs, it will cause the chain to slip off pretty easily.  Its called cross-chaining.  In other words, your chain would be in the little ring up front (on the far left) and your chain would be on your little ring in the back (on the far right), or your chain would be on the big ring up front (far right) and on the big ring in the back (far left).  Cross-chaining will also cause your chain to stretch and wear the teeth on your gears down.  A good rule of thumb to go by is this:  If you have a triple-chainring (you do) and the chain is on the small (inside) chainring in front, use only the inside three or four inside cogs. If the chain is in the middle chainring in the front, use the three or four center cogs. If the chain is on the outside chainring, use the three or four outside cogs.

Fourth.  You said you took it to a bike shop (and for what its worth, I trust bike shops zero - I have been taken by several of them, but what choice do we have), which makes me think it may be one of the reasons above.  However, the bike shop may not have fooled with your front derailleur, checked the chain length, or looked into the front drive - and I bet they didn't.  Did you tell them you were having issues with the chain coming off?  If not, and if nothing else works, it may be worth having someone look at just that problem.  There are a lot of slight adjustments that can be made to the front (or rear) components that effect the chain switching gears.   My first bet is that it is the adjustment of the front derailleur that is needed and that's the first thing the bike shop should look at.  If it is your front derailleur and it needs some adjusting, it should be a very simple, quick, and cheap fix.  I would recommend Carolina Tri just because they deal with Specialized and work on a lot of them.  Go in and ask for Ben and ask him to take a look at the front derailleur and see if it needs an adjustment.

Try the first three things before anything else and see if that helps.  Let me know and I'll do all I can to help you out.  Glad you're liking the bike...it looks pretty good considering how damaged it was in Joshes picture, huh?