Posted a couple of weeks back about this problem, but wasn't aware until recently that it seems to be directly related to heat. Have loosened the kickdown cable pretty much all the way, and have also been running an AutoRX treatment for the last 700 miles, but no results yet. Anyone have any ideas?:

When first driven, car shifts through all gears almost imperceptibly, and goes into overdrive easily. About 10-15 minutes into highway driving, or after periods of heavy stop-and-go traffic, it will either drop out of overdrive with light application of the gas pedal if on the highway and never go back in, or simply just never go into overdrive if gradually approaching speed from a stop. This happens more rapidly on hot days, much more gradually on colder ones - seeming to indicate it's temp related. If cruising (at this point, grinding along in 3rd) for long periods at lower-load highway speeds (lower ambient temps, flat or downward grades), it will occasionally pop back into 4th - again seeming to indicate that cooler operation allows overdrive. Trans will also take a few seconds when initially shifting into Drive when hot, but goes in instantly when cold. Shifting through all other gears is still smooth when hot, though.

I know the relay is good, fluid level is good, and I intend to do a flush once I've left the AutoRx in long enough. Kickdown cable operation appears smooth and unrestricted. I was under the impression that most solenoid, dirty or low-fluid related problems work in reverse - no overdrive when cold, but then ok when warmed up. My next inclination is to do the operation on the solenoid described in the FAQ, but there's something nagging me about the trans. taking those extra seconds to shift into Drive when hot. Just wondering if that could be related to the no OD when hot also, and if it's an indication of something possibly more serious than just the solenoid, like the valve body or something....

Sorry for the long post! Any insights or similar experiences greatly appreciated.