Auto gearbox problems

[costellon]

Hi, I have a 96 3l auto Landcruiser Prado tx, jap import with 116k km. When on a long run it intermittently chooses not to go into top gear at all and sometimes does. I have found that when it chooses not to go into top, the revs being at about 3,500 rpm at 140 kph, the temperature guage whizzes up into the red after about five minutes and if I don't slow down (to about 120, to drop the temperature) it overheats. This is my first automatic and don't know whats going on. My mechanic maintains that he changed the transmission fluid about 3k ago (he possibly didn't) and I have dipped and checked the transmission fluid and it does not smell of burning and there is no water present. What do you guys reckon it could be ?
I intend on changing the transmission fluid myself, could you please tell me what the capacity is and what fluid I should use exactly.
The problem only exists when the vehicle is hot, It does not happen when i'm commuting around the city.

Thanks.