I bought a used 2005 Kia Rio a few months ago (manual transmission). I had no problems for the first few weeks. However, one day I started it up and when I tried to drive it, the car felt like it was bogging down, for lack of a better term, and wouldn't go above like 10mph. It felt like when you shift to too high of a gear while driving slowly, though that was not the case. The car had some shuddering and wouldn't get up to speed.
When we took it in to get repaired, it started driving just fine on the way to the auto place (of course)! They took a look and couldn't find anything wrong. I have had several such incidents since then, but am never able to get it to the auto place while it is still lagging. Every time it happens I am on the way to work so I don't have time to try and get it to the auto place while driving the 10mph (or sometimes not even that fast). Also, it just randomly goes back to working fine.
One car place said it might have something to do with the weather, like maybe when it's wet out something in the car gets wet that makes it bog down, then dries up and it is fine. However, this problem only happens some of the time after snow or rain, but it seems plausible.