I'd swap the torque converter. I had my torque converter go out. Same thing. Driving down the interstate and it'd randomly rev up and not engage. Few hundred miles later and it sounded like metal pieces rolling around in the converter while just driving down the street. I swapped it out and no issues with the tranny after that. A torque converter is only $100 or so, obviously getting to it is a ***** but it's a relatively easy fix and the parts are cheap. I replaced mine around 180k also. I wouldn't drive it anymore if you can help it, but I think you'll be fine if you swap it out and in doing so the whole tranny will get flushed with new fluid too. I'd have the mechanic drop the transmission pan first, that will show if there's any huge chunks of material and then he can swap the filter out too. Even if the tranny is bad, tranny's are pretty cheap. I actually have a tranny with a low mileage TC for sale that I can't even get rid of, though I'm in Utah.