1984 6.9 with c6 won't stay in 3rd gear, keeps down shifting no matter the load.
When I start in drive I have to manually put in 2nd for it to shift.
Then I put in D but it won't shift into 3rd until I drop down into 2
nand force it to shift, it shifts for a moment but qucikly
downshifts back to second no matter the load. It was shifting
hard now not at all. Please help!!