Torque converter probs

Have an '89 xj and the torque converter is locking and unlocking intermittently only in the hottset part of the day. Never in morning, never at night, and not even while towing, unless in the heat of the day. It doesn't happen every day and only happens in 3rd and 4th gear only between the speeds of 40-55 mph. When it does act up I simply turn the ignition off and then on again while driving and it seems to drive normal again. Took it to a tranny shop, and to the local Jeep dealer and they say most likely electronic but that they cannot diagnos the problem. Any help would be much appreciated.