1994 Monte Carlo, 198xxx miles. 3100 engine. Having a problem where the torque converter locks and unlocks at exactly 70 mph. It will lock at the normal points but as soon as it hits 70, it will unlock for about .5 seconds and then lock for about .5 seconds, and will repeat until you are under 70. If you lightly step on the brake pedal, which would unlock the torque converter, it will stay unlocked and you can speed up. But it will continue the cycling if you let go if the brake pedal. Anyone heard of this before?

 

Welcome to the forum!
With the amount of mileage on your Monte.. When was the last time you had the trans maintained? Fluid filter change etc? Sounds to me like solenoid is starting to fail... Unfortunastely on these transmissions the solenoids are in the transmission. and not outboard.. like days gone by...

I would recommend taking it to a trans shop and have them use there code reader to be sure exactly whats going on with the lock up mechanism.
You standard code reader cant read the trans codes... They have a "special" one designed just for transmission issues like yours.. Please be sure to et us know what the fix was/is... So in the future your info will also help others!
Thanks for joining the MCF Family!

I'm sure someone else will pop in with other suggestions/ideas.