This Explorer XL, either a 92 or a 93 with the 4.0 and an automatic, 150k, has a strange problem. When it has not been driven for a while, the transmission will randomly slip into neutral while driving, usually uphill. In order to get it moving again, it is necessary to shift into park (neutral is not sufficient), race the motor for approximately 10 seconds, and then shift back into drive. Shifting directly to park and then back into drive is not sufficient either. The problem is completely eliminated by allowing the motor to warm up until the thermostat opens before driving it, i.e. it has never slipped out under this condition. This has been going on for at least 10,000 miles. It acts the same in 4x4. The transmission has been maintained with new fluid as required.
Any thoughts? I'm wondering if there are any known computer or control system issues out there, but I suspect the torque converter. The problem is mostly annoying rather than serious, so more invasive diagnosis isn't really justified. I'm also wondering if it is likely to be doing any damage when it pops out of gear or not.