No Warning Lights

It's probably not the lack of premium gas, but rather that use of a lower-grade gas that doesn't have detergents in it. Over time, this can cause carbon build-up and fuel-system problems. It's not for sure, however. This kind of symptom can have many causes, but a good and cheap first place to start is to run a bottle of Chevron Techron Concentrate Plus with the next tank of Brand Name, high-quality gas. See if it makes a difference over the course of a tank or two.