I replaced trans and torque converter but it keeps popping out of gear won't shift into 2nd or 3rd what's the problem?

Is this an automatic, or manual transmission? If it's an automatic trans, chances are that it was driven hard, which stressed and tore up the torque converter box, and thusly began to strip your transmission. If these were replaced with used parts, say, from a junk yard, or other dead vehicle, it could be causing stress on your engine, because these components didn't "grow up together". Thusly, the non-gearing up. If this is a manual, or standard transmission, I would highly suggest looking at, and maybe replacing? the clutch-plate, and throw-out bearing.