Repairing Mac128K board - memory problems

Hi, All, A list member recently sent me a broken Mac128 logic board. I quickly found a bad solder joint from a previous repair where one of the original 64Kbit DRAMs was replaced with a socket, but I'm still having issues. The Mac does "bong" and I do get a "Sad Mac" with a hex code. It claims it's the same chip that is now socketed (G8). If that DRAM is installed, I get "Mac in Jail" bars. If that DRAM is removed, I get black pixels where that chip was. The other odd symptom is, between the bars, a semi-regular sparse grid of on-pixels. It's not a perfect pattern across the screen; it's mostly on the right side, and not always top to bottom in a column. It does look regular, as if it's every 64 bytes or some other power-of-2, and 5% of the pixels blink as if they are changing in software. I know that Apple-branded DRAMs of this era are dodgy. I could just clip them all out and replace them with fresh, *tested* chips (I have a DIP DRAM tester), but I am concerned that the fault is elsewhere since a known-good DRAM always produces a stuck-on bit (the "jail bars"). That smells to me like a bad gate upstream of the RAM field. Looking at the available schematics, unbuffered DOUT D0-D15 go several places - a pair of '244s to buffer the data bus, a pair of '166 shift registers for video data out, a pair of '161s for sound data out, and a 16R8 PAL named ASG that appears to be related to sound and diskette PWM. The DIN side of the data bus connects to the '244 bus buffers, the CPU, the ROMs, the IWM, the 6522 VIA, and the Z8530 SCC. I would think that if that side of the bus had a stuck bit, the CPU would be fetching bad data from the ROMs, so there wouldn't even be a Sad Mac. I can do some catastrophic repair by blindly pulling all the RAM and replacing it, and I can even pull the CPU and socket it so I can stick my Fluke 9010A on the board and do some serious memory tests and exercise the bus and poke at it with an oscilloscope. What I'm asking for at this point is any information that can help me target my efforts. It seems like this sort of fault would have been common in the day, but mostly when I google "Sad Mac" or "jail bars", I get stuff about replacing individual RAM chips or changing SIMMs in an SE/30. So anyone have any advice that could help me not remove and replace 320 solder joints on a multi-layer board? Yes, I can do it, but it's a lot of time to do it well. I'd just like to get this old Mac working with an original board (I have working 512K boards, but the point is to have an Original Mac in time for display next January). Thanks for any suggestions, -ethan