If it's not showing any ram or rom errors on bootup but the graphics are still glitching, either with vertical lines through the textures or polygons warping then it's most likely bad Fujitsu mask roms on the rom board. I've fixed several before by replacing bad roms with flash chips programmed with the relevant graphics data but it's a lot of work, primarily finding out which chip has gone bad. Blanket replacing all the rom chips isn't really feasible.
Lesser common faults are bad ribbon cables or dry solder joints on the mask roms.
Finding replacement boardsets in recent years has become very difficult, as people are more likely to hang on to them due to these failures. I know I would!