2001 Mitsubishi Eclipse misfire

My eclipse has developed a misfire after it has been driven for about an hour or so! After the engine really warms up the rpm gauge will spike slightly and the car will shutter a little bit and it slowly get worse the further you drive it. My check engine light went on yesterday so I took it to my mechanic and he said the computer said that the error code was a bad sensor in the distributer so he replaced the distributor1 but this has not solve dteh problem, the car runs great other wise, but when it gets real warm it will misfire!