I've noticed on my long rides that when I'm about 7 miles from the finish I lose a lot of oomph. I don't know if it's psychological, biological, or both but either way it happens on most of my 30-40-50 mile rides. Sometimes I stop and get off the bike for a brief rest and then proceed. Other times I just slow my cadence and silently curse the lack of performance. Just wonderin' whazupwidis.