Have you looked at the battery terminals for build up of fluffy white/blue
matter?  Battery acid can eat through the contacts after a few years.  I
high drain device, such as the starter, can physically shift the contacts
enough to either allow enough current draw to run the starter or impede it.

You have a 12 year old vehicle and the battery (probably replaced at least
once) could have corroded the contacts at the battery to the point that you
are only getting partial contact.  Also, battery acid can run down inside
the of the insulating material of either the positive cable (one to the
stater motor the other to the power distribution module) or negative cable
(typicaly one to the engine block, another to the body and a third to the
frame) and corrode those connections as well.  Acid can also "pool" inside
the cable at a low point and corrode the cable there enough to prevent the
starter from running, but I doubt that is you problem as is sometimes
starts.

If it is caused by corrosion of the contacts at the battery, the starter, or
the ground straps, it may not be evident by a visual inspection, but that is
where I would start.  Look for oxidized copper (i.e. the color of the Statue
of Liberty) or week/loose connections/cables.