It's when I try to actually board the plane that I get pulled aside, 100% of the time. Actually, it really happens at check-in -- my boarding card gets flagged with some kind of special marking indicating that I must be hand-checked.

Ahh, ok. I got randomly selected flying out of COS, but never on the way home. My friend who flew recently got randomly picked for this search at the gate as well. Thats really odd that you have been nailed every single time.