I last changed my transmission fluid around 200k miles, and since then I've had a recurring occasional slip from a dead stop, as well as an occasional hesitation when downshifting to pass on the highway. After doing some reading, it seems the symptoms may be traced back to my using Dexron 6, when the vehicle calls for Dexron 3. Dexron 6 is supposedly backwards compatible, but after doing some research it seems the viscosity rating is lower for Dexron 6, meaning the fluid is thinner, which can apparently mess with fluid pressure in higher mileage transmissions.
Today I changed the fluid back to Dexron 3, and will drive it for a while to see if it solves my slip/hesitation issue.