2003 shifting issues
 

Hi All,

my son has a 2003 accent it has 100k on it. he brought it in for a tranny flush and now the friggin thing does not shift properly. It goes through the gears but will not downshift. It is as though the thing is stuck in overdrive. The other day he was driving at about 60 ad he heard a bang, suddenly it was down a gear revving at 3k. He has had other issues with it, at time it was like it would just shift into neutral and the only way to solve the issue is to turn it off and restart.
Anyone have any suggestions as I am stumped, there are NO codes no nothing

Anyone have any ideas what the problem may be