I recently noticed my tranny dropping in and out of lockup but it was very rare and at higher speeds (between 55 and 70). I posted a question regarding it a couple of weeks ago. Anyway, as this was rare, I was not too concerned about it and decided to do some research using the search function.

At the beginning of this week, I cleaned the barb fitting on top of the tank because it was blocked and my fuel tank was sustaining vaccum. Since I have done this, every time I drive the truck in overdrive and reach 45 to 50 mph, the tranny drops in and out of lockup ... what's the deal??? Is it just a coincidence that this occurred??? Someone help, I can't stand it!

 

I would recommend cleaning battery terminals and reset apps. There is a pcm reflash that might help as well, TSB 18-02-99.