Unless it's a the last couple minutes of the half they basically don't even stop the clock on 1st downs anymore, maybe for a few seconds at the most. And then I've seen on many, many occasions a runner clearly go out of bounds and the official spotting it is signaling for the clock to keep running. They probably do it just to "shorten" the game a bit, there are so many TV breaks its a joke (absolutely brutal on fans at the game), I guess if they operate the clock as the rules dictate every game would be 4+ hours.
Hard to be consistent unless you do things consistently, but the clock is clearly operated differently at the end of a close game than it is in the middle of the 1st quarter.
