[bstrom1953]

If you have a tachometer, take the car briskly up to 50-60 mph. After you reach a constant speed, watch the tach and it will suddenly drop between 200 and 500 rpms. That's when the torque converter locks up. If it has in fact locked up, you can unlock it simply by accelerating sharply. You will see the tach jump back up as the car begins accelerating.

If you don't have a tach, you must listen to the engine and feel a slight surge when the converter locks. Remember, the converter will only lock up when you are at "cruising speed", not under acceleration. Then accelerate sharply and you will hear the engine increase in rpms and feel it unlock as the car accelerates. It feels just like having another gear, like you are driving a four speed, but it's not.

One question: After you restart your car, when you put it in gear, does it try to take off with a jerk or lurch and then die? That's what they do when the converter is stuck in the locked position. You feel like you need to push in the clutch (if was a stick) but there is no clutch! You also said the problem shows up after longer periods of driving. Those transmissions were notorious for locking up after they got good and hot. The plastic in the solenoid swells from the oil and the heat swells the solenoid steel plunger. Together you have a sticky solenoid. Let the car cool and the solenoid slides freely and the converter unlocks.

FYI, the converter will always unlock when you step on the brake or let up on the throttle. THen when you resume crusing speed, it will lock back up.

My car has a 3T40 tranny, turbohydramatic 125.

Hope this helps.