Why is engine-transaxle disengaging?

What is your question?

Engine-tranaxle disengaging happens at highway speeds, when gaining elevation or moving into a curve, for example. Seems to me to be at the final phase of automatic transmission shifting where lock-clutch would kick in. I am at highway speed, either on cruise control or accelerating slightly, when suddenly the engine-transaxle disengages, the engine rev's, I take my foot off the accelerator (if not in cruise control), the engine rev recedes, and engine-transaxle re-engage. It is happening with increasing frequency, most recently about 5 times in 2 hundred miles of travel. Another symptom is that on occasion, when I am slowing the car to a stop or near stop, the engine starts to stall and "engine service" light blinks momentarily. Have read this could also be a torque converter lock-clutch problem. What does this problem indicate to you? If the lock-clutch or torque converter is the problem, can the torque converter be serviced at less expense and complication than the rebuilding or replacing the whole transaxle? My shop mechanic (3 times) and dealer (1 time) both could not pick up the problem (no computer codes registered, and I sense they are unwilling to test ride 100 miles on the highway to try to experience the problem; my local mechanic drove it around for 24 hours). I am grateful that neither went on a wild goosechase to address the problem and work, but I am disappointed that neither could diagnose the problem. The problem is real and has happened probably 20 times over the last 1,000 miles; also passengers in the car have likewise experienced it.