Converter Lock-up problem

Hi, I have a 1996 Chevy 2500 4wd 120000 miles 4L80e trans. My torque converter keeps coming in and out of lockup while driving in any gear and any speed. Here's what i've done:
replaced the brake light switch
Trans. Professionally rebuilt
checked fluid.
The snap-on code reader showed the brake switch was constantly going on and off so I replaced it (no results) This problem was present BEFORE the trans build. The problem was intermittent (maybe once a month during a 15 min drive, to all the time now. No codes have come up. The problem occurs whether the truck is cold or hot, but seems more noticeable when cold. The trans. Shifts perfectly.