Losing gas mileage - little power

I have a 2006 Hyundai Elantra with 65,500 miles that is losing gas mileage and a little power. It began about 3 weeks after having the 60,000 mile timing belt replacement and water pump replacement, although the dealership doesn't think the replacements are the cause of problems.

3 weeks after 60,000 service, gas mileage suddenly dropped from 29 miles per gallon to about 24 mpg. Dealership recommended changing from cheaper gasoline to brands like Shell/Chevron. For a few tanks, this brought mpg back to 29 mpg, then began dropping again.

I put a bottle of STP fuel injector cleaner from Walmart through and mpg went back to 29 for a couple of tanks, then started dropping again. While STP was going through system, a few times while driving on the interstate I could feel "something" seem to open and give a slight increase in power and gas mileage. But the next day, I could feel "something" seem to choke down with a slight loss of power and gas mileage. It began to drop again.

Dealership said put a can of Seafoam through. Immediately mileage dropped to 23 mpg for the initial tank, then went back to about 28 mpg for next 2 tanks. Now is dropping again and currently at 26 mpg.

The Check Engine Light has never come on and dealership says it is probably due to ethanol in gasoline even the better brands. However, I know my car is not acting like it always has, and seems to have a mechanical/part issue since I could feel something seem to open and later choke up.

I truly can't afford a "hit and miss" part replacement series. I have always kept current on all maintenance and recommended replacements. Dealership says spark plugs aren't due for change until 90,000 miles.

Can you help me?