On a recent road trip I had been driving for about 2 hours and stopped for a few minutes. As I started back onto the road the transmission would not shift into overdrive. This transmission was replaced by the dealer some 35K miles ago. I changed the fluid some 15K miles ago. The color, smell and level of the fluid were normal. I pulled over, selected "3" on the shifter and proceded on to the next rest stop. After about 30 minutes, I got back on the road and with the shifter in "D" again, everything worked normally. Two days later I'm on the road again, this time about 20 minutes and when I stopped at a toll booth and began to pull away, the car seemed like it was trying to start in high gear. Once again I stopped, selected "3" and took off fine. This time I manually shifted to "D" at about 3K rpm. It went into overdrive and shifted up and down as my speed dictated. The problem continued at each toll booth, traffic light, etc. I made it home and parked the car for about 2 hours. When I drove it, everything seemed back to normal. I thought there might be some temperature considerations, but, the temp gauge never fluctuated from it "normal operating temperature" position. I am about to change the ATF to Castrol Synthetic. How do ATF change intervals differ between synthetic and Mercon III? Heeeeeelp! Thank you.