From my point of view, the circulation of current within your cells didn't go very well. That's why you observed high current at the beginning, which dropped quickly. It is like you are charging a capacitor... It seems to me that the TiO2-dye part worked fine. The dye were generating photoelectrons. However, somehow, they didn't get "reduced" sufficiently, which caused the current down.
It might be the redo migration within EG or the Graphite that can't sufficiently reduce I3-. (suggest: using Pt, instead of Graphite)