Last winter I had the problem of the tq converter unlocking and re-locking at steady state speed in the 42-47mph range. I had some helpful posts and solved the problem - electrical interference due to a wire in contact with alternator case.

https://www.dieseltruckresource.com/....html?t=222513

Now I have a new problem, but similar, and need to know if the above fixes still apply to this new problem. My torque converter unlocks and relocks at all speeds cruising, accelerating, etc. Constantly. As soon as it locks, it immediately unlocks (all speeds above 42-47 range). I did not do the APPS reset, nor the gear hunt fix from ATS. Could this be a failing pressure govenor which was replaced about a year ago?

 

Don't know if this will apply to a 99 model or not, but we had the same problem on our 95. Would unlock and lock at all speeds and for no apparent reason. Also the cruise control would not stay on for more than about 30 sec at a time. I got to nosing around and found that the brake light switch under the dash was kind of loose. I popped it back in place and slid the adjustment as far away from the brake pedal as it would go and that cured it. I guess the brake pedal was bouncing off of the switch enough to make up the contacts and take out the cruise and t/c lock up.

It might be a similar set up on a newer truck.