My friend has a 1999 Taurus with the AX4N automatic. In cold weather, it shifts fine into overdrive, but when it gets warmer, it will only shift into OD when the engine is cold, and after it gets into operating temperature, it will refuse to shift into OD, unless you almost completely take your foot off the gas pedal, and then it will sometimes shift, but will immediately shift down again when you press the pedal down. I have tested all the solenoids with a 9V battery. All click fine and have the same resistance. I took off the valve body and examined the retaining clip (according to the TSB), which appears to be perfectly intact, and also checked the wiring connections, changed the fluid and filter, cleaned the cooler fins. All other gears and TC work fine. Does anybody here have any good info to check for?