burnunit

Hello all, new member here, I posted this is other sub-section but this thread might be more appropriate. Did a search but couldn't find a recent thread with similar problems so here we are. Bought a used 1999 Tahoe with transmission that was supposed to have been rebuilt for my son. He drove for about 10 months had intermittent issue going into 4th and down to third and back. Suddenly I get call that its not moving at all, research after the fact tells me limp mode, I figured trans went bad and swapped for one out of a donor that was wrecked, didn't know about limp mode yet. Swapped in the trans and truck drove better than the rebuild, actually went through all the gears, son went to get inspection but needed to put miles on for computer after it reset. While driving it suddenly went to having nothing when put into drive, google research lead me to limp mode and this site. What could cause the trans to suddenly go to limp mode while in the middle of driving? Bad ecm? or bad shift solenoid? Looking forward to getting some feedback and hopefully being a productive member to the board Thanks Burnunit