[weblacky]

While perhaps not practical, given that people have chimed in with similar issues, I'm wondering if you're dealing with a structural issue. If you can find a pin/trace or Basic IO pinout of the XMAP2 chips, and place an oscilloscope at the same input of each chip (to verify signal), then later at the output (whatever form that may be) of each output pin (perhaps input of next stage instead). If they work in parallel (not in rapid series) then the failing unit should be visually different from the output of the others (lack of output I'd think).

I'm wondering if you're not dealing with a cracked BGA joint (chips look BGA, I don't have a pic of the other side to know for sure). You mention having check all caps and resistors...so what else is there beside the ZIP memory modules?

If you could find a way to attach a 30awg+ wire or something to an output point of each pipeline and just look at the signal (without even understanding it), you'd assume 1 out of the 5 doesn't look like the others. Then you can decide if it's chip or joint.

From what's being said here there are two failures (complete and partial), this makes me think of a connection issue, rather then a power issue. I'm not an expert but the picture of the board you provided doesn't appears to have much in the way of power boost/buck circuits...so I don't see much in the way of voltage conversion on this board. So considering a power issue, wouldn't the symptom would be more widespread (affect all pipelines at the same stage)? I don't see a huge VRM section or anything like that...so unless they somehow hid it inside an IC back then (doubtful) I think they are working off a common rail.

Consider finding someone with a really good BGA rework preheater station, and reflow the expected XMAP2 chip and see the affect.

Food for thought, considering it's not a idea I saw proposed in this thread. We see this MUCH more today than back then, but considering I don't see any heatsink on these, it could stand to reason that repeated localized heating and cooling could cause thermal expansion and cause a cracked/poor (high resistance) joint? Which would affect signals.

Also consider hitting it with a FLIR camera and check if one of the 5 XMAP2 chips (both sides) is hotter/colder than the others by comparison.