Corolla / Matrix 2009-13 :: 2010 S - Auto Trans Downshift Delay And Jerk

I have a 2010 S with an auto trans and 68,000 miles. I've never replaced the fluid or filter. Over the last 10,000 miles or so, I've noticed the following in regards to shifting.

I will being on a merge ramp to the highway rolling along and waiting to pull out into the flow of traffic. I'll be traveling along at a constant speed, say 30 mph, with constant pressure on the throttle waiting for my opening in traffic. I will see my opening and floor it. That's when (sometimes) the car seems to get very confused. It doesn't shift back right away, it lingers in 3rd (or OD) and picks up a tiny bit of speed. Then, it hesitates with no acceleration, and then roughly shifts to 2nd or 1st. It feels more like it forces itself into gear and, once in gear, it accelerates like it should.

The biggest issue for me is that this whole process takes about 3 or 4 seconds. That's not very long, but when you're trying to merge onto the highway or cut into traffic, 3 seconds is the difference between being safe and a close call.

Transmission fluid appears to be full, is a nice red color, and doesn't smell burnt.