'96 AFT auto gearbox issue

[tim]

Hi All,
Back after a few years and needing some help pretty quickly.
Can anyone help me pinpoint why, when driving uphill and under load, the autochange is fine for 1st, 2nd, 3rd (I think) and then seems to rev up past 4000rpm then reluctantly "tries to change to 4th". If I back off the throttle, it does drop in gear, though when revving again, it is comparible to a "slipping clutch-free revving" of a manual car, with no discernable power output or acceleration. Also,
if I am going downhill and put my foot down at about 60mph, the same thing happens-free revving.

There are no noises, bangs or anything else-the van also seems perfectly fine at lower/around town speeds and driving conditions. Been told "AT oil is low/old (it is old I admit), pressure seal gone, higher range clutch (?) gone etc etc" I just want the guys that know Bongos best-you, to advise before I put it in the garage.

Any help really appreciated.
Thanks in advance,
Tim (S Wales)