I bought this truck used about 6 months ago. At least a year or more before I purchased, the previous owner had the transmission rebuilt and an air to oil transmission cooler installed to replace the original radiator bottom tank oil cooler. Until yesterday the transmission has performed flawlessly. Yesterday I noticed that the Torque Converter would unlock under very light load or going downhill. The tach would indicate engine RPM dropping, then coming back up. I could feel the slightest disengagement and engagement as it occurred, and I also noted that the trans temp was running about 6 to 12 degrees F higher than normal (I have a trans temp gauge installed). The day before this problem, I had engaged the 4-wheel drive and disabled overdrive. When the problem occurred, the truck was in 2-wheel drive with overdrive enabled. When I returned home from my trip I checked the trans oil level and smelled the fluid. The level was correct, the fluid was clear and light red, and there was no burnt smell. There are no transmission diagnostic codes.

I'm looking for suggestions about a possible cause and a good place to start troubleshooting.

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