As it goes away...it may be braxton hicks contractions..i was told that labor contractions are usually regular and increase in intensity slowly.Good luck
I would just call and tell a nurse about it. Being that far along you never know what it could be. Strong pains obviously mean something is going on especially for it to be timed like that.
Since it always goes away, I'd keep your eye on it but try not to worry. Mention at your next doctors visit. If this increases in frequency or intensity, then call the doctor about it now.