Early design 700 tranny will upshift to 2nd gear while the shifter is in 1st.
You can do 1 of 2 things;
Install a larger low/reverse boost valve (.296 diameter)... or install a TransGo shift kit...
700- 2 & 3 kit , will over ride the command to upshift from 1st to 2nd gear while the shifter is in 1st gear.
IF you have a late 1987 & newer 700 tranny, perhaps the shifter adjustment is out or your tranny builder did something I am not thinking of at the moment. Could be mismatched parts
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