Hi All.
Been long time since on this side.
Having recent tranny rebuilds help.
YET

At 30 - 45MPH The torque convertor wants to lock up.

YET the Speedometer needle wiggles at those speeds.
By 45 MPH it locks "well" (better than at that 30-45 range).

WHAT CAN I DO?
Replace speedo cable?
Replace Speedo head (Have that option in garage)
Pull plug to "lock/not lock"? (NOT seeking that option, really)
INSTALL A SWITCH WHEN I WANT TORQUE CONVERTOR TO LOCK (after 45 MPH for sure!)!
Comments, suggestions,
Before "the good stuff (rebuild is a year old now & freshly serviced & told good & it is)" is ruined by this TORQUE CONVERTOR LOCK UP/NO LOCK UP DANCE!

My vehicle is a 1985 Pontiac Firebird powered by a 3.4 w/700R4, cable driven speedo with this "YelloW box" on back of instrument panel.

put a new speedo cable in and that should solve your problem. I believe that box on the back of the instrument panel is the vss sensor. I think the older converter lockup was computer controlled as well so replacing the cable with a new one should solve your problem.