1999 F250 Transmission Problem

We have a 99 F250 with an automatic transmission that has a hard shift in 3rd gear, only third gear and only up shifting. It has been serviced every year for the last three years and nothing every shows up in the transmission fluid it has also been hooked up to the scope numerous times at different dealerships and a transmission shop and no codes ever show up. There is nothing consistent about the hard shift other than the gear, it doesn't do it all the time going up hill, towing, under hard driving conditions. A torque converter problem has been suggested but ? Any ideas ?
Thanks /ubbthreads/images/graemlins/shrug.gif