Have you had any luck fixing yours. It's been almost 2 weeks and mine hasn't glitched since so the deduction that futzing with the motherboard (i.e. I had to remove it) likely moved something that caused one of the socketed chips to lose full contact. Grabbing the 9-pin signal off of MC1733 for color composite wasn't causing the issue.
The only thing I have done so far is to open the QL and re-set the chips in their sockets. No joy. Unlike you, the problem is not intermittent. The QL is simply totally dead with the exception of the power LED and Hermes that clicks when a key is press until its buffer fills.
I have been using QLs since the first days and have had many problems with them, bad contact, etc, but every time something could be seen on the screen (garbage) often accompanied with terrible sound coming out of the speaker and wild running microdrive motor. It is the first time the QL is dead like that.
Next step is to remove all adds-on and try again with a BBQL config. However I am beginning to fear of a bad ZX8301, which would explain the lack of image and processor activities.
I would gladly accept some guidance as to what to probe to check the health of the ZX8301 and other support chips.