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[Claude Stander]

Hi All. I am in South Africa. I have a 97 Fairmont with 415 k on the clock. The auto box runs fine when cool and normal. All of a sudden the engine revs spin up and I have no gears. At times it is as if the box resets to 2nd gear and gets traction with a lot of squealing in the box.
Sometimes by selecting 3rd gear I can get to my destination without too much spinning up of engine revs.
If the vehicle has been standing for a while I get in and drive with no problem.
I suspect it is temperature related, as the problem is more prevalent in town/short distance driving.
I replaced the filter and oil, and still have the same problem.
Does anybody have any idea what it could be? I was told that one or more of the valve body seals may be blown/ cracked.
I can get a service kit at what the car is worth, but to replace what I have is going to astronomically expensive, so at this stage I am seriously considering repairing the box.
Does anybody have a parts manual to strip and repair the box?
Any advice would be appreciated, as I can't seem to get any assurance that the repairs will be successful.
I have a 1929 Model A that is so much easier to work on and am in the process of restoring a 1057 Austin Healy 100/6, so i have reasonably good mechanical skills.
Look forward advice.
Regards
Claude