Engine "surge" during constant speed

[xdarkxfirex]

The yaris has somewhat of a naturally funky acting tranmission. We bought a lease return last year from a private dealer with around 122k, and I put around 30k on it. When cold, it takes a bit longer to shift to 3rd from second. Since it only has 4 gears, I assume that it could be causing somewhat of a problem. When I go uphill or downhill, I find myself jumping between 3 and 4 alot. I can even manually change it many times from 3 to 4.

A few months ago I did a drain and fill with full synthetic atf at around 50k. No difference, and there wasnt metal shavings or anything. I believe it's just the design. You shouldnt worry to much about it. If you get high or low revs, just give it a little more or less gas or change it manually