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Commodore 128DCR: VDC (80 column) display issue
I just got my C128D working again (see original thread here for overview) and so I thought to load a few programs to take the repaired internal floppy drive for a test run. That's working flawlessly, but what I discovered was a problem with the VDC. Some text on the screen was distorted in places:

As you can see from the pictures and descriptions, this problem only really shows up in 128 native mode. CP/M mode seems to be working just fine, and the 40 column display doesn't have this issue either. I also know for a fact it's not my monitor, as the same monitor works perfectly with my flat C128. I speculate that it might be some sort of issue with a RAM chip, but wouldn't that affect CP/M mode as well? Any suggestions on what to look at to get this resolved would be much appreciated.
Further tests
I dug out my Commodore 128/128D/1571 diagnostic and service diskette and manual and ran some tests on the VDC chip/memory (PASS), internal disk drive MFM and GCR R/W tests (PASS) and disk drive logic tests (PASS). My only theory at this point is that there is some sort of noise/interference that occurs when the disk drive is being accessed that causes this video distortion on the 80 column display. If anyone has any pointers or suggestions on how I could narrow this down, it would be much appreciated.
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