Terrano changing revs.

The vehicle is an imported 1990 Terrano
It has 140,000 on the clock.
Has never really towed any more than a small trailer rarely.
It is a Turbo 2.7 diesel Engine TD27033604T 4X4 4 door 3 speed automatic wagon.
It usually doesn’t go into top gear until about 75 Ks/Hour
The vehicle seems to change revs a lot as if it is slipping back to second.
Sometimes I am sitting on 100Ks at 2500 revs
The next thing I am sitting on 100Ks at 3000 revs.
Sometimes It will run at 100Ks at 2300 revs
You can hear the engine change revs but sometimes the rev counter changes accordingly and mysteriously sometimes you hear like the engine change revs but the rev counter doesn’t move. I have always though it was getting to top a bit on the slow side like revving out a while in second.
Now it is like you are doing an even 100ks on a smooth road and suddenly sounds like engine revs up but rev counter doesn’t indicate this.
I really don’t know anything about these turbo charges I don’t even know how to really tell if the turbo is working or not. I paid over $100 for a manual and when I looked to that all it said was if turbo charger fails – replace. LOL.
I have checked transmission fluid and if any thing have slightly overfilled to no avail.
Any ideas would be welcome. nasa@netbay.com.au