When I make my yoyo strings, loops of thread start popping out after 5-10 minutes.
Uhh, like randomly down the string, individual threads are coming out? You might, and probably are, making them wrong if that is the case, since they should be all a continuous length of thread twisted into a string.
You should be twisting them one way, folding in half, and untwisting in the opposite direction, nice and easy.