how can i fix my auto gearbox in my 1993 isuzu big horn 3050 cc derv?

my gear bog keeps changing up and down its gears on hills some times its like it go's down two gears into second at 60mph? that makes the moter rev at 3500 rpm, iv tryed takeing it out of overdrive that did nothing, it still droped down into second, this only happens if im giveing it some welly(over half throtel) and only on hills, is this the way they are ment to work or is it a fult in the box?? i still get around 19 mpg, a uk gallon that is! some times i wished id stuck to a stick shift! as the auto big horn is a real dog to get moveing, any idears?
on this one, is it just how they are or is it busted??? coz if its busted, so will i be trying to get it fixed! ££££££££££ OUCH!