2003 Chevy Impala The engine is hesitating at high speeds

Ok, I have a strange problem. First off I replaced the fuel filter today and it did nothing, also replaced plugs last summer. The vehicle idles fine and when I rev the engine its also fine. But, I get on the road and when I am trying to excelerate, I have little power. If I push on the gas real hard and the engine is cold it misfires. I know for sure I must have a bad thermastat since the engine is running to cold but I believe this is unrelated. I am not sure about getting computer codes because the engine light isn't on and I thought that that would indicate something is wrong. Any suggestions before taking to Pep **** and having my credit card maxed out. Thanks