Buddy's did the same, tried a lot of things. One mechanic said to pound the dash with his fist, he did and it would work again. That tells me bad ground connections around cluster. He did replace cluster and is working fine now but the disconnecting could have been helpful too. How many times in older cars just wiggling the fuse cured issues.
Common practice on those 70s Dodge trucks with the fuse panel on top of the dash under the glove box door (spilled drink)