Chuckles1

Sometimes when I shift the 10 speed RD to a lower gear, it shifts and then immediately drops back to the original gear when I release the lever. It's only occasionally, but obviously frustrating when you're starting up an incline and lose momentum.

Not sure if it's technique or what. I may be inadvertently moving the upshift paddle when I push the downshift lever, or the lever isn't sufficiently separated from the paddle and grabs it enough to move them both at the same time. Never happens on my other bike with older Tiagra 9 speed levers.

My only idea is to slip some fine sandpaper between the lever and the paddle and buff to make sure they're smooth. Not crazy about removing finish though.

Any other ideas before I try mine?

dsbrantjr

First check to make certain that the inner shift wire is not fraying inside of the shifter; getting the cable out before it breaks is WAY easier; check this regularly and whenever there is a shifting change/problem. You might also consider a solvent flush/relube of the shifters; perhaps the ratchet pawl is not catching reliably, letting it drop back to the smaller cog.

Iride01 

My son's Tiagra 4700 had a frayed cable up in the shifter after only a couple thousand miles. My 105 5800 had a frayed cable after three thousand, maybe four thousand miles. It's something you won't usually be able to tell without pulling the cable end out of the shifter a few inches. If you wait too long, you'll have more trouble getting it out.

The symptoms you describe are classic for a frayed cable. However certainly there are other causes too. But for me, this has been the cause the last two times I dealt with the issue you describe.

Get the idea of sand paper out of your head. Especially if you are just guessing at a cause.