If you don't have a replacement, you could swap the two RAM chips over (socket them after desoldering and removing them) and see if the graphics problem changes.
You could also check the logic gate on the chip select logic chip for the two GRAM chips. Most commonly this is driven by U17, a 74LS86 quad logic chip, using an XOR gate on pins 11/12/13 (output/intput/+5v). Chip select input comes from ADDR8 bit line, pin 14 on U5 (GROM), and other address lines and ENABLE and READ/WRITE comes from the same chip, so you could try swapping that chip too. And check both GRAM chips are getting a solid 5V at pin 22 (inspect 0.1uf caps C7 and C18).
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