You need to measure your chain for stretch, it may be too stretched and no longer meshing with the gears properly. There are tools for this or you can do it with a ruler--one foot of ruler should exactly match the distance between pins on the chain; if the pins are not lining up with the marks on the ruler it may be time to replace. If you do replace the chain, cut the new one to length according to instructions rather than according to the old chain length, in case it was too long.
Check your high and low limit screws on your front derailleur--perhaps they are set too far apart.
Also try tightening the "B" screw of your rear derailleur to put a bit more tension on the chain. The "B" screw is underneath the derailleur hanger and can be screwed in from the rear of the bike.