Is it worth repairing my 11 year old Toshiba 3357DB television?

Recently the picture has been intermittently dropping down from full screen to a thin line running horizontally across the screen half way up the screen. I have given the set a percussiuon adjustment (I hit it) with some success but it still returns to the thin blue line eventually, sometimes after a few hours and sometimes after a week or so. Now I can't get it to budge away from the thin blue/bright line in spite of repeated bashings. I have removed the back of the tv twice, 1st time I cleaned the dust out and re-seated as many of the connections as could be removed. After re-assembly there was no change initially from the the thin line but a swift tap on the top of the box saw the picture return to full screen only to disappear after about 7 days. 2nd time tv back off,re-seating connections and re-assembly did not produce a full screen picture in spite of repeated beatings. Sound is still ok but I am concerned I will do some more damage by repeated beatings. I have a vague notion (from older valve tv's) that the "tube" is unserviceable and this renders the tv an unviable option for repair. Your expert electronic advice would be appreciated by a grubby gas turbine engineer !