2002 Ford Mustang No "get up"

I have a 2002 mustang, v6, and recently it has no power. When I hit the gas, it seems as though the gears are not shifting, I have to let go of the gas for the rpm's to drop back down and when it does that it jerks. There have been occasions when the car will die out (or vibrates/shakes when I come to a sudden stop. The check engine light comes on every few weeks but I never catch it on at the shop. Please help! I don't know much about cars, but the shops can't seem to figure it out either. One mechanic says maybe an intake issue? Not sure. Any help would be appreciated.