There are so many different clues here, this thread is an example of what can be done on these boards when people get together and spread nothing but there actual experiences, as opposed to flaming other members of the board.
The universal problem is that bandaids are being used over and over again instead of extinguishing the culprit. Some muscle testing and theory could be used to show you the problem you are having with your lower abdomen. Over the course of your life your central power distribution station has been slowly moving away from that sacred spot of your body that affords perfect geometric efficiency, a bit below and behind your navel, to where it resides now, which is most likely very near to if not in the exact location of your current abdominal injury.
Try performing a muscle test to see if the horns of the downtown Eugene trains have kept me up in these wee hours for nobody's benefit...
A. Lie down on the floor in the anatomical position; arms to the side, and stomach/abs, palms of hands, forehead, and tops of pointed feet are in contact with the ground.
B. Begin to flutter kick, as you should expect a swimmer would.
C. Raise your head up so that if a mirror were before you, you would find yourself staring back in/through your own eyes. (Keep flutter kicking)
D. Perform a motion nearly identical to a swimmer's stroke by extending one arm out ahead of you then sweeping it back under your chest; obviously, repeat with the other arm as well.
D.(b) You could also choose to mimic the action of a runners arms, and find the only thing possible is quite similar to the stride Micheal Johnson has made popular.
I'll assume you are a bit wobbly and may find that you roll towards that injury, the power source. This should come as no surprise as energy grounds itself through the quickest line available.
The person who commented that sneezing was especially excruciating seems to have tied this together. It is quite rare to find someone who can induce sneezing at their will. In the world of sport, one could reason through observing elite athletes inconsistent performances that it is also rare for someone to induce, through will, a state of pure flow where the athlete finds/loses? themselves in a zone. This is important to note as sneezing is an action so unconscious yet perfect that we can safely conclude it to be a brief moment of aforementioned pure flow. During that brief moment you are not choosing where your power comes from, your body intuitively knows where the most energy can be generated from and sends out an incredible burst of energy that has such an effect as to create an internal hurricane when measuring air speeds exiting the nostrils.
The post about swimming backs this up as well, as that person seems to get better once in the pool and feeling properly "stretched out". If you don't swim from where your core was designed to be, you will find it difficult to hold a straight line, the legs may fall a bit causing increased drag, breathing out one side of the mouth may be found to be much easier or feel more natural than breathing out the other, etc. It sounds like swimming would be the best way to rewire your grid and "extinguish the culprit".
...why a runner, performing but one of their inherent capabilities, should be viewed as superior to the well trained triathlete is beyond me.