Atari TT displays random graphics and crashes during boot

[Roberto]

Hi everyone,
Here I am again with yet another problem. I've been trying to enjoy an Atari TT that I recently bought for about 10 days now, but every time I solve one problem, another one immediately appears.
I decided to start a new topic because this issue is different from the previous one.
Previously, my biggest problem was that the computer would not reset when I pressed the reset button. I eventually discovered that the cause was a faulty 74LS05. That issue has now been resolved, thanks in large part to the help of @frank.lukas.
The new problem is shown in the attached photos. The TT no longer boots at all. Depending on the power-up or after pressing the reset button, I get different corrupted screens, such as the ones shown in the photos. Quite often, the computer doesn't output any video signal to the monitor at all.
After fixing the reset issue, I reinstalled the motherboard onto its metal chassis, making sure to put the insulating cardboard sheet back in place. That's when these new problems started. I'm wondering if, while reinstalling the board, I may have accidentally shifted the insulating sheet, allowing one or more solder joints on the underside of the motherboard to touch the metal chassis.
At this point, I'm completely out of ideas.
Any suggestions would be greatly appreciated.

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