I pulled the Guralp breakout box from the rack, and it's sitting on the EE bench here. The game plan is to check out the Gur2X channel.
Rana and Steve have been investigating, and found that the X channel has been funky (which has been known for ~a month or two) when the seismometer has been plugged in, and also when the seismometers have been unplugged, but the box is left on. The funkyness goes away when the box is turned off. Since it's not there when the box is off, it seems that it's not a problem with the cable from the box to the ADC, or in the ADC channel. Since it is there when the box is on, but the seismometer is unplugged, it's clear that it's probably in the box itself.
Preliminarily, I've connected a set of BNC clipdoodles to the input testpoints, and another set to the output. They're both connected to a 'scope (which is on it's battery so it's not connected to any Ground), and when I tap on the circuit board the input trace is totally unchanged, but the output trace goes kind of crazy, and gets more fuzzy, and picks up a DC offset. Koji is concerned that some of the big capacitors may have an iffy connection to the board.
Investigations will continue Monday morning.