How can I isolate cause of torque converter lockup engaging and disengaging?

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Noticed this happening when A/C on and traveling fast enough to be locked up. Tach was fluctuating and could feel it locking and unlocking. Seems to do it right when maintaining speed. Now it does it without A/C on. Noticed that I can get it to lockup by reducing throttle position and load, but as soon as I increase TP, it unlocks. Too much of this gives light and code p0740. Did not notice the lockup problem until started using the A/C this summer. Tranny also will not shift into 4th gear in Winter until I am 5 miles up the road. Used magnetic heater on pan and decreased to 2-3 miles. I did get a "fluid temp..." something code at one point during the winter. Tranny also fits one of the brake band adjustment symptoms in the FSM. Shifting from 2-3, it "boggs down" like it's going into 4th, then "lets go" and revs through 3rd and shifts into 4th normally. I can avoid this if I let off the throttle at the right time for the correct duration, but it's tricky. FSM said to back off on brake band adjustment. I have not had any work done to the transmission. Bought used with 77000miles and now has 95000. Trying to figure out if it's feedback from sensors or actual problem with TC, solenoid, fluid pressure, or PCM.