Gauges/dashboard going haywire? 14 Equinox

My wife's equinox has begun to do something interesting.
2 days ago, her Service Stabilitrak light came on, and shortly after, Service
Power Steering flashes across the gauge and her gauges began to fluctuate
rapidly, from where they currently were down to 0 and back. (RPMs, MPH,
digital speedometer, engine temp)
The battery tested good, as well as the alternator.
I did clean some corrosion off of the terminals, and the grounds that I could
get to. The car operated fine for about 3hrs of drive time (45mins to work, 45
mins back, 20mins to the store, 20 mins back, and about 40mins drive
today.) When she approached a light, those same lights kicked on, and the
gauges began to go haywire again.

What else can we check? Note that I was not able to get to ALL of the
ground wires as they seem to be buried beneath a bunch of hoses, or maybe
I'm not following the correct path.