the ground is usually a central point. but in this case, you would need to pull the dash out, unless you can see up from the bottom. you want to find a set of wires that are attached to the frame or body somewhere as a common point. if the connection is tight, which i suspect. I am suspecting that there is a rubbed wire in the back of the instrument cluster that is bare and when you go over certian terrain, the wire is no longer grounded and everything works. check also around the steering column shaft and see if any wires are sitting/rubbing the shaft.
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