Automatic 3rd gear slipping when hot? Help please!

[Futurespec]

I am having problems with my 1993 NSX automatic. Just had gearbox rebuilt but the car still slips in 3rd gear when hot. Appears OK when cold but when normal to hot there is no line pressure increase under acceleration.

Do you have any suggestions to the probable cause or have you experienced any similar problems. Any help and suggestions would be much appreciated.

Thanks in advance,
Michael (UK)