faulty toshiba

Hello there,i was wondering if any one can help with a problem i have with my tv.its a toshiba 36zp38b widescreen crt,about 2 years old and until recently utterly reliable.the initial problem was with the screen breaking down into lines and was diagnosed as a faulty scan converter,toshiba kindly replaced it free of charge.within a few days similar symptoms appeared as well as the picture "flicking" and shimmering.the dfs scan converter was changed out again but the symptoms persist and the engineer is at a loss as to what could be the problem. any advice would be appreciated