After taking a look at the schematics for a few minutes, I would suggest checking out the 4 74253 mux's. for problems. I assume UF8, UG6, and UG7 are working correctly as the system is getting its vblank and hblank signals. I also assume UE8, UC6 and UC7 are working correctly as UE8 (shift register) is shared between the two RAM chips and each RAM chip is outputting 4 bits....

Chips of interest:

PALs:

UE6

UE7

Mux:

UD8

UB8

UC8

UA8

bigmessowires' A1 assumption for the 16 pix of garbage does sound correct when you consider that the VRAM and RAM are seperate.... so i would check out pins 9,10,11,12,13, and 15 on UD8