3rd gen auto transmissions will stop working between 75-150k miles if they don't receive regular maintenance (less with abusive driving). This is due to the cummulaive effects of being overheated. The fluid gets oxidized and temperature sensitive internal seals become damaged. Fluid goes where it shouldn't, and there isn't as much to push on the clutches. The clutches begin to slip, which causes more heat, which overheats more parts, the clutch matierial starts coming off its backing plate, which clogs up the fluid passages, so the planetary gears don't get enough oil and they burn up and on and on until the thing stops working.
When it dies you need a new transmission, but you can't buy one; there aren't any more. All there are are rebuilt ones. Rebuilt isn't remanufactered either. All that gets replaced are the soft parts like seals, bands and clutches. This is why you generally can't get more than a 12 month/12,000 mile warranty on a rebuilt. They will not last as long as a new transmission, and not very long at all without regular maintenance.
The maintenance that has to be done is to keep fresh fluid in the transmision, and to keep it from getting too hot. The way to keep the fluid fresh is to get it changed (all 7 7/8 qts.) every other year, or whenever you notice a color change. The way to keep it cool is with an external cooler. (If you get a rebuilt you HAVE to get a cooler because the cooler in the radiator is contaminated with crap from the wrecked trans., and can't be properly flushed out. It's either this or a new radiator.) Synthetic ATF is the best bet.
Where did I find this out? From the Infinity forum on Yahoo. There is a part owner of an independent Infiniti repair place on the forum. Early Infiniti Q45's have worse transmission problems than we do, and J30/M30's have the same (same transmission). What I've related, is what they've found out over the years to greatly extend the life of the transmissions.
If you want an even more reliable transmission, get in touch with Level 10. They have a couple of different kits to beef it up.

 

i couldent have said it better myself!

 

Thanx for that info, I am scared as hell that my tranny will die and i try to do everything i can to prevent that....so that info was very informative. also well put

 

Thank you again for that well thought out and informative info.

I change my 91 SE auto tranny's fluid every 10K miles. Its got 111K miles right now. But I will now give more TLC to this car.

 

Aloha! I am writing from Hawaii. I just bought an '89 Maxima GXE two days ago and am noticing some problems with the transmission shifting down and up between 25 - 50 miles an hour. I bought it from a dealer and the guy showed me a receipt of him just having the transmission flushed thoroughly. He said he thought that some sensors(one for throttle position and the other for some type of temperature?) might need replacing to fix this. I will look into it. I don't think there is a separate tranny cooler installed. I will go out tomorrow and purchase and install one. Thank you so much for the informative post.