Youve all probably been asked this, ut my problem is as follows:

I have a Tn exec EFI

On first startup I drive off, sometimes revs will skyrocket and take awhile to change gear,
when it finally decides to change gears who can feel and hear a big thud as it changes into 2nd......

Also sometimes it will do the same thing but THUD into what I think is 3rd and stay in that gear. If after it gets stuck in 3rd, I pull over and switch the car off then on again, It will drive fine, changing gears and all.

Does this sound like the Auto is on its way out, and should be rebuilt? How much to rebuild?

or is this something else?

Another ques, the aircon works perfectly but when you turn it ON revs drop then come back to normal, but you can feel the power loss difference when it is on, is this normal?

Any suggestions or experiences would be appreciated, because this car is clean straight no rust and just hit 125,000 original klms and I would like to keep it.

Welcome to AussieMagna :D
That is pretty bazaar as I had the same problem in my third gen with the gear changes... I don't any more though... I did get the gear box serviced once but it didn't do much... but I believe in my case the computer was at fault and after a few resets and relearns it was ok.
Is this a new car for you as in you have just started driving it? cos if it is that could explain it.
As for the air con... not sure mate unless the belt is too tight or something... not much of an air con expert.

Power loss with aircon is definately normal! It certainly zaps a few kw's out of every car when on. The revs dropping then stabalising, I am guessing that that is just the car's way of adjusting to the aircon

for a 1st gen a rebuild on autos can range from $1200 to $1600

but as for the a/c yes thats all normal the revs drop becuase your putting alot more load on the motor and you ISC motor brings the revs up a few seconds after so it doesnt strain the motor, and on my TN when u go up a hill with the a/c on u can feel it go alot faster when it clicks off plus it only being a 4cyl you have to expect that

Sounds like your auto is on the way out, 1st Gen autos are pretty ****, might even be worth investing in a manual conversion ;)

Sounds like your transmission's gone the same way as mine.

It could be the fluid needs replacement. Use the proper fluid, otherwise all hell'll break loose.
However, if that doesn't clear it, the failure is a clutch, usually the rear, that is at fault. Mine's been doing that for the last 70,000km, so it may last, but then again, it may fail next week. The only repair is replacement.

Incidentally, I bought the car due to the tranny problem. It's the only thing I've not actually replaced yet.

Thanks for the help, I might get it to a Tranny shop to investigate further,
I have already done a basic Auto trans filter kit added the new fluid, it stopped it for awhile and it also made gear changes almost unnoticeable, but I guess it was to good to be true, the problem is back.

Ill keep you guys/gals posted on the news from the Tranny shop after diagnosis.

Thanks again.