My car is doing something weird...?

Hyundia Elantra... 2006... The car starts to "putter" almost like it's going to stall, when I press on the gas at about 40miles an hour.. between 1 and 2nd shift.. it has a hard time getting up hills now too.. it putters a lot.. The engine light comes on from time to time too. What do you think is wrong with it? And what do you think the cost could be? Also, I have kept up on oil changes, and checked fluids, all seems to be okay.