OK. On your power board you have a metal pad marked VS-TP, do you see this?
Carefully set your meter to the highest DC voltage range for example 500V, 600V or 1000V. DC is marked by a solid and dashed line. If it has autoranging capability, set it to autoranging volts DC. Red probe on meter goes in the jack marked VDC or VOLTS and black probe goes in COM. Make sure it is not set to volts AC, which is marked with a squiggle like this "~".
Place the black probe on part of the metal chassis -- somewhere it will go safely. For example the wall mount bracket is a good place. This is the cold ground.
The red probe should then touch the test point. Make sure you can touch the test point without slipping (do this with the set turned off first.) Make the measurement on that point with the TV running. Try testing on a dark image -- a dark movie, or the "No Signal" pane. Then try testing on content which normally makes the problem appear. See if there is a change. The voltage should not drop by more than about a volt and it should match the panel sticker within 2 volts (208 to 212 V.)
Careful: You are working around a power supply which is LIVE. The board exposes high voltages, not only the Vs but also the mains voltage. Do NOT touch anything on the power board while it is plugged in, or shortly after turning off the set. Many of the heatsinks on the board are live and can carry deadly voltage. Always have another person in the room to supervise in case you make an error.