Just a couple of comments. Looks to me your temps did drop, which you corrected. My thinking is that occationally you have both the IHS and the Heat sink a little off. Sometimes they concidentally match. For example one concave the other convex. If you just lap the heat sink you'd end up with a worse match and higher temps. The other possibilities have been mentioned. Poor lap job and loose fit as the Ultra-120 mounting is poor to begin with.

If you do get higher temps after lapping then check the lap, then modify the mount to get better pressure. I'd probably just put some thermal tape on the back plate. That would move the back plate away from the board and give you a tighter fit. If that doesn't work, then lap the IHS or remove it.