recommendations for transmission maintenance

have inherited a family owned (and well maintained) 93 trooper
based on meticulous records, engine oil, filters, plugs, harness, axles,
boots, differentials (dual since 4wd) and finally the transmission, have all received a regular inspection or parts or fluids change
car runs and sounds like new but am a little concerned over the transmission as it does sometimes lock into 3rd gear which is a known GM transmission feature that is designed to protect the transmission if something is not right.
the fluid is clear, no odor, pan dropped and filter replaced couple months ago, replaced about 9 quarts of the total fluid capacity at that point. there was hardly anything inside the bottom of the pan, a little amount of residue at the bottom which I think to be some wear from the surfaces and carried by the fluid to the bottom of the pan.
transmission shifts smooth, makes no sounds and drives for a month without incident, then all of a sudden when coming to a complete stop at a red light, the red transmission indicator (check trans) comes on and the 3rd gear lock is engaged. it is reset by simply turning the car off, restarting engine and all is well again for another month.
if I replaced it with a rebuilt transmission it would cost about 1200-1500, parts shipped to me and I would have to ship my existing tranny back within 30 days. if I took this to a local transmission shop, I can foresee them very quickly spending that same amount in diagnostics and not sure what else to consider.
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Ignore the problem. Wait until it gets to be bad enough to worry about. Then drop the rebuilt in.
DO NOT buy a rebuild today and keep it in a crate until you need it. If you do this and there is any problem with it, you may find your warranty has expired before you need the thing. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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Scott makes a good point. A transmission shop I used to deal with in Houston told me they never rebuild trannies and keep them on the shelf because there is a risk that they will deteriorate (seals, rubber parts). They always rebuilt them to order. If you buy one and keep it in a crate, you may actually be creating a problem.
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