2002 Ford Ranger • 3.0L 6 cylinder 2WD Automatic • 100,000 miles

After vehicle sits for a long period of time (8 hr) on a hot day, it does not want to 'drive' vehicle forward. When shift lever is placed in 'D' and foot is taken off the brake the vehicle will move forward slowly (no foot on accelerator) but when the accelerator is pressed the vehicle will not pick up speed as expected. Its as if the transmission is not engaged. After the shift lever is fiddled with, the transmission will work as expected.
This problem does not exhibit after short periods of sitting, or on cooler days. Ideas?
July 10, 2014.

Is the trans fluid clean and full? Have you checked the linkage to make sure it is engaging into the correct position?

The A/T fluid was recently replaced ( fluid exchange 2000 mi. Back) along with a new filter. I think it was the first time it had ever been done looking at the fluid color. It didn't have any burnt odor though. I haven't checked the level, but feel confident its at the correct level. I think the linkage is working properly, but would welcome some guidance on the proper way to verify its working correctly.
Thank you.

Jul 11, 2014.
Just check to make sure there is no play in it at the transmission. Since it works when you move it around, it sounds like it may not be engaging properly.