Sometimes when im set on cruise at about 70 mph my check engine light comes on?

i recently did a tune up. sometimes when im set on cruise at about 70 mph my check engine light comes on.at no other time does it come on. also, i need to replace the gas cap because its allowing air into the gas tank. do you have any suggestions for me on what the solution might be for me to fix the car. Thank You, Melissa Any info. would be helpful. Im not familiar with american cars...