Chain suck is very frequently caused by one or two things. As the chainrings wear, they develop burrs that do not allow the chain to release very smoothly. These burrs then start wearing micro grooves in your chain and further increase the likely hood chain suck will occur.
The intense pressure of pedaling often makes chain suck happen exactly when you don't want it to.
We would suggest replacing the chain, and carefully filing any burrs on your chainring so it's smooth. If this doesn't do it, it may be time for a new chainring.
Chain suck will also often occur in extremely muddy conditions. This is sometimes unavoidable and the only solution is to clean your bike.
Also, try backing off on your pedal pressure slightly when shifting your front derailleur. Extreme drivetrain tension can sometimes cause chain suck during a shift.
As with any maintenance issue, it is generally best to take your bike into your local Specialized dealer to have a mechanic inspect the bike.
The cause and solution of chain suck may not be listed here and would need to be identified by a mechanic. If your chain rings and chain are worn to the point that they need to be replaced, it may also be time to replace the cassette as well. An experienced mechanic will be able to tell you what will be needed to get your bike shifting properly again.