Glad you got it fixed. I have a 240 with 84mb Ram with a crappy screen artefacts issue. Thought it might be vram but might recap first and hopefully fixes the issue.
This look similar to what you were getting? When I connect external display I get the same output. Tried reseating everything. Cleaning etc. but no different. Where did you get the vram from? Mine is a 3400c 240
I have the exact same problem. Started my 3400c today after it being in the cupboard for a few years and getting the same issue. when I change the mode to 256 colors I get less issues. Feel like doing another? 😂