I've noticed the same thing w/ my '99 GS300 too. Happens like 2-3 times a year, and I notice it only when I'm in a parking lot driving <10mph looking for a spot. Not too worried about it, just sometimes breaks the smoothness of the ride. I'm no engineer, but seems like the engine speed is borderline in terms of 1st to 2nd gear change, so when it does shift it does abruptly.