jerky speedo and erratic shifting on hi gear, (from 3rd to lockup)

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Speedo needle goes crazy from 25m/h and faster jumping up and down at the same time 3rd gear goes to lockup also shifting up and down gears ATF level is normal, 1st and 2nd gear work fine so does speedo at those slower speeds 2nd gear engaging at 15mph 3rd at around 20mph (maybe too soon?) I adjusted the TV cable and planning on changing SSV before moving forward to a more expensive repair. Question: Could a shortcut or faulty connection be affecting TCM or other elements involved in tranny work?