I had the exact problem you described, except between 3rd and 4th gears. When I inquired about it on this site, I was told the cable inside shifter was likely frayed. I started to pull the cable, but noticed it wasn't frayed, and has only got about a thousand miles on it. What I did notice is the original Tiagra cable looked like it was greased, all except the last inch and a half, the part inside the shifter. As a learning experience, I tried rubbing a little grease onto the ungreased part, and reinstalled it. For the first time in a year it's not dropping shifts. I don't know why, and await the onslaught of posts on why you shouldn't do what I did, but for now I'm happy.