Anyone had a hip flexor injury?  I had pain during the Tokyo Marathon which intensified toward the end and then went away after about 10 minutes after I finished.  It has not been a problem since then until today during a 16K run.  Applied ice and the pain eased (the end of the run helped too).  I know rest may be the key but what is odd is that it comes on and the “hibernates” for a while even though I don’t rest.  This is not an unusual pattern for me.

The RICE method should be your first reaction. RICE stands for Rest, Ice, Compression and Elevation.
To learn a bit more on this method http://myfitnessdepot.com/sports-injuries/injury-first-aid-the-rice-method/

Another strategy could be to modify your running style. Recently, I have begun using a technique called ChiRunning ( http://www.chirunning.com ). In essence, ChiRunning aims to lessen the forces that most runners experience by introducing a slight forward lean and a biomechanically efficient flow to their running style. It advocates running with relaxed lower legs and using a mid-foot strike to avoid the braking and impact of heel strike. Since adopting the ChiRunning approach, I have found running even more enjoyable, not to mention less strenuous.

That is a strange pattern. Review your training log for the last week or two to see if there is anything that could have brought it on.
And yes, changing your shoes and running form is quite possibly a good idea.