UC6 and UC7 are video DRAM, mine are soldered into the PCB. They have been known to go bad. But also on the suspects list is the row of video address and data multiplexers/shift register UA8 thru UG8 that are vulnerable to electrolytic leakage particularly from SMT cap C2. Inspect carefully for signs of moisture leakage and trace shorts/corrosion. If the video ROM UK6 and video timing generation PAL parts UE6, UG6, UI6, UE7, and UG7 are in sockets, they should be carefully (keeping ESD in mind) removed, leads cleaned/straightened, and reseated, just in case a bad socket contact is causing the problem.