Bad ram can cause this, or possibly a problem in the timebase board.
In either case, you will not have access to the parts or technical info required to properly fault find the scope, well not unless you are, or work for, a Tektronix agent or authorised service centre.
My best advice is to take the scope to an authorised calibration centre and let them deal with it for you, that way not only does it get properly repaired without damaging anything else, but it also gets a recalibration in the process. It won't cost the earth to repair, certainly way less than a quality replacement, but it may end up as scrap if you touch the wrong thing inside it.
rgds