I have a 1999 mercedes benz c-class. The transmission has gone faulty. I havereplaced it with a used one. The car would run fine for about a few hours and then get into the limp mode (no gear change). I took it back to the mechanic who programs it. I got it back and it ran again fine for about a few hours again eventually going into the limp mode. This has been happening for about six times now. Can can anyone help with some form of advice please?