Hello, I am John, a Ford dealer technician.
I hope I can help solve your problem quickly and accurately.
First off you need to check that all your linkage is connected and working properly. The regular shift linkage and the kickdown linkage. Both of these can cause problems. If ok you probably have a sticking governor valve. If this is the case you will have to replace the governor assembly. This is located on the output shaft inside the tailhousing. It is held on by two large phillips head screws. You can purchase these at http://www.wittrans.com/ This company sells quality trans parts at reasonable prices. I have been using them for a good while with great results. If you still have a problem after this (probably not) then you have an internal trans problem and are probably in need of a overhaul.
If you have any more questions please feel free to ask, it's what I'm here for!
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