Don, are you keeping track of the weather? If so post the numbers on temps, humidity, baro. press and so on I have a freind who was having similar problems, his car would slow down .1 (1/10) to .25 ET (in the 1/8 mile) in the afternoon after our lunch break and drivers meeting. He lean the carb down 1 jet size(Holley type carb with aftermarket parts) and the car would drop .1, he leaned it down one more jet size on all four corners and the car would not drop much more then .03 Cool, thick engine oil, trans fluid, rear end grease and water temps can and do affect the way the motor and car runs