1994 Chevy Corsica Cutting out, hesitating

Just started about a month ago, Service engine soon light came on and started cutting out and hesitating and bogging down at 40, 50, & 60 mph etc. It starts and drives fine as long as I am accelerating or slowing down. However when I just try to matain a constant speed, such as 60mph it starts cutting out and bogging down untill I either go faster or slower. I put new spark plugs and and air filter in but it did not help at all.