That alternating back and forth is really common in IBSers.After being backed up for a few days the body can "over flush" to get that out and that can set you up for the next round of constipation as you get too empty.Often taking an osmotic agent every day so you go more regularly (or fiber if that works for you) will prevent the diarrhea. Taking meds to prevent the diarrhea tends to be counter productive as that tends to over constipated which sets you up for more diarrhea to get that out.After lunch is pretty common as that is one of the times the colon is more active. It is most active around the time you get up and then again after every meal.