I had a Dakota with the NV3500, it did that for about 7k miles before I finally got annoyed enough to take it in. Turned out the 1st gear detents were shot and the shift fork was bent. Either of those could have caused the other. So, it needed the first couple gears rebuilt, didn't have a problem after that. I know it isn't exactly the same transmission, but the design is similar. Mine was popping out from a dead stop, and only if I tried to put it into first after I stopped, when I'd let out the clutch it would throw itself out of first. If I pushed it into first withe the truck rolling I wouldn't have the problem.