I think the last time he lost clean two PPVs in a row dates back to before he became the top face and won his first WWE Championship at Wrestlemania 21.
But two in a row losses last happened in 2011. He lost four out of 5 back to back PPV matches including MITB & SummerSlam to CM Punk and Hell in a Cell and Vengeance to Alberto del Rio with a victory over Del Rio at Night of Champions in the middle of these PPVs. None were clean, however.
I know that in 2008, he lost cleanly three ppvs in a row: to Triple H at Night of Champions, then JBL at Great American Bash, and finally to Batista at SummerSlam. I think he was because he was filming "12 Rounds."