After moderate, hot weather driving, transmission hangs in 3rd gear.
It eventually drops to 4th with throttle coaxing.

in as much as I know, there is no 4th gear in '91 with the 3.1...but there is a Torque COnverter Clutch ; when it kicks in after 3rd gear at steady speed and light load, it can give the impression of being a 4th gear.
Anyway, there is a protection mechanism that prevents the TCC from applying when transmission oil is too hot. What you describe can suit that explanation. ...but there may be other issues too. I'd rather hope for the first explanation.

in as much as I know, there is no 4th gear in '91 with the 3.1...but there is a Torque COnverter Clutch ; when it kicks in after 3rd gear at steady speed and light load, it can give the impression of being a 4th gear.
Anyway, there is a protection mechanism that prevents the TCC from applying when transmission oil is too hot. What you describe can suit that explanation. ...but there may be other issues too. I'd rather hope for the first explanation.

Thank you as that does explain the situation. I was thinking it needed a tranny cooler. Probably not worth the investment with 175k on it.