Ok so I started trying to fix another Robo board set last week. Upon initial power-up, I thought it was odd that the rug screen was jittery, almost like a 60-cycle RFI situation. All the components are wired up on my desktop, and I'm using a 13" crt as a test monitor. It stayed in rug and repeated, and it tried hard to boot, but it only drew one-half of a black screen each time. Then things got weird. Eventually the black background became orange. It continued to rug and draw the background from left to right, orange each time. At one point I ran my fingers down the side of one of the 74153's, and by doing so, I could affect the draw rate of the background! I could run it back and forth with my fingers. WTH? So I replaced 3 of the four 153's and—the jittering stopped and the game booted! I thought I had it licked. The next day I walked in and powered up, and it booted. Sweet! That day was the last time it did that. Ever since, the jittery screen returned, the rug freezes, and the background attempts to draw in orange. (The orange NEVER turned to black.)
So over the last 3 days I have replaced all the 166's, the 7489's, the 9316's (with 161's), the remaining 153, a 139, the decoder sockets (I tried another pair of decoders to no avail), moved some rams around due to the random ram errors on the ROM board's segmented display, even tried another ROM board, replaced the 4 caps in the power section, and have new headers and plugs on everything—and NOTHING has changed. There are new AA batteries in the holder, there is no battery acid on this board, and the ribbon cables are fine. I only replaced the 166's in line I because 3 of the 4 were running pretty hot. The 7489's and a few others had blackened legs and had obviously run hot for some time.
I know that 7474's are ridden hard on these boards but I haven't touched them. I don't like randomly swapping chips; I'd rather have an informed direction to go before I do anything else. My voltages are fine. I'm getting tired. I followed the flowchart in the manual but the 139 I replaced at 4G was the only chip that was a touch suspect.
Once in a blue moon, the sound board emits a solid high-frequency tone that won't go away until you turn everything off. It may come back, but you never know when. Wtf?
Any ideas, anyone?
I couldn't post a vid of the jittering and the periodic re-draw attempts, but look at the rug pic and imagine it shaking so rapidly that your eyes ache and you get a headache.
A few other notes:
this is a Revision B board!

I am using the original Williams (rebuilt) power supply.
I resocketed and replaced the 6809 first thing.
As stated earlier, I swapped ROM boards temporarily and nothing changed.