S197 Transmission

Hi . My 05 Gt has just had the transmission rebuilt due to slipping between 2nd - 3rd ( Servo bores worn , common known issue ) but now is doing something different and the shop that rebuilt it is having trouble working out the issue.
The transmission is operating normally until you start going up any incline while in OD ( Normally on highway at 60MPH ) , it will change down to 4th at any sign of incline but then will change back to OD straight away , then back to 4th , then back to OD, all the way up the incline.
Has anyone had a similar issue ?
Maybe it's the torque converter ?
Thanks Mike
 
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