1990 speedometer problem

[drt0800]

Noticed driving the last few days that the digital portion of my instrument cluster is acting up. Works fine, hit a bump in the road and it becomes garbled. Hit a bounce again it clears up. Don't really know if it's coincidence. Last night I parked and it was fine. This morning I started it and it was garbled again (I'll attach pic). Now it's not clearing up. I can't run the codes because I can't read them in the cluster. Could this be a grounding issue? Speed sensor? I did get a check engine light on Monday but it went out and never came back.
Any suggestions on troubleshooting this would be great. Thanks

 

[vetteoz]

Works fine, hit a bump and it becomes garbled. Hit a bounce again it clears up.

Typically indicates a bad connection somewhere on the supply or more usually grounds