4L80E Torque Converter Issue??

[99Express]

Hello, new to the forum. I seem to have a transmission/torque converter issue on my 99 Express 2500. Since the kids have grown, I mostly use this van as a tow vehicle for our Airstream trailer. We just returned from a short trip with the trailer and the torque converter did not lock up as usual. This caused higher than normal transmission oil temperatures.

In the past, when traveling at speeds above about 45 MPH, the torque converter would lock up, keeping the transmission oil temperature moderately low. On this trip, throughout most of the first day, it acted normal. Near the end of a 40 or so mile leg on I-5 at around 60 MPH, the torque converter would randomly unlock. This continued to happen where either the torque converter would just not lock up, or unlock randomly (even on level, constant speed, areas where there were minimal or no throttle changes).  When towing, I am very aware of the torque converter locking as the transmission oil temperature stays low (well under 200°F).  I installed a transmission oil temperature gauge many years ago, it is the type that has the sensor in the cooling line to the cooler(s), so it reacts quickly when the torque converter locks up (or unlocks).

On the drive home today (about 140 miles), the torque converter rarely locked up.

The transmission fluid is in good shape with no odor or discoloration. Since we use the van for towing so much, I change the transmission fluid about once a year (just draining the pan and refilling).

So, I wonder what is going on...
- Torque Converter needs replacement??
- Transmission Issues (I hope not)??
- Some Control Computer Issue??
- Something Else??

The Van: 5.7L engine, 4L80E Transmission, JET Performance Chip Reprogrammer (premium fuel), 4.10:1 Rear Gears.

Any insight would be much appreciated.

Thank you,
David