The same thing happens to me w/ Kevin Brown. I have a very solid pen, and by limiting Brown to 6-7 innings, especially when I have big leads, I figure I can squeeze a few extra starts out of him. However, he's tossed all of his complete games on the road.
One strategy that seems to work is to specify a 4-man rotation on the days your ace will start. If, say, his maximum use is 35, and his recovery is 9, the robot knows he has to come back w/ 3-days rest and will pull him after 32 batters (35-32 = 3; 3 + 27 = 30).