I have a '78 280 CE, 6 cylinder, double overhead cam, with 190,000 miles. After driving for 30+ miles at 55-65 mph on a straightaway and then decelerating to make a 90 degree turn, an ENORMOUS cloud of white smoke blows out of the tailpipe which lasts for 8-10 seconds. After having embarrassed me totally, it's fine until the next 30+ mile trip with a turn. I can drive in town at 35-45 mph for hundreds of miles with no problem, but as soon as I drive for 30+ miles at 55-65 mph on a straightaway and then decelerate to turn, the smokescreen reappears. Any suggestions on what to tell my mechanic to check out?