There can be a variety of factors in play here, but more often than not they tend to be either 1) the amount of focus put forth towards each shot, or 2) the rhythm or playing style of your opponent.
The first point is a common story around pool rooms. It seems like we play our best when we're outmatched, doesn't it? And some of our worst, most deplorable play comes at the hands of a much lesser player? To put it simply, the mentality shifts from "I really need to focus or I'll get my a$$ handed to me" to "I don't need to try as hard since I'll probably get another shot later." Consciously or unconsciously, the change happens. The solution here is to develop a routine (I'm sure SPF and Eberle students can be more specific) such that you perform each shot with the same level of dedication, thought, and focus. You don't have to pressure yourself, but you have to make sure you're not mailing in your work.
The second point here can be a little more subversive. This may not happen with everyone, but I've seen games turn into "shooting matches" where one player's propensity for shooting at the hole or playing aggressively in turn lights the fire inside their opponent to do the same. This also goes for a player shooting quickly, playing more safeties, banking instead of other options, etc. Again, the solution would most likely work back to finding a routine (that is, a conscious list of things to do) that works for you and adhering to it. You can't completely discount the effects of your opponent on your play, but you can do your damndest to ignore the unimportant parts.
I hope this helps shed some light on your dilemma. The first step to fixing a problem is completely understanding why it's a problem in the first place. Good luck and good shooting!