Yet another area to check...The video from the logic board passes thru a single 74LS38 buffer, U1, on the analog board (and pullup R1 and series terminator R30) before being passed on to the CRT mounted video board. U1 also buffers the vertical and horizontal drive into the analog board. R28 and C22 decouple the +5V power to U1. A bad U1 or bad decoupling could cause some video artifacts as well as weird interactions between video, vertical and horizontal drive signals. Even if the +5 looks good on the power supply connector, it could be less good at U1 pin 14 either in DC value or noise contamination.