96 - RPM on my gague cluster is malfunctioning

[unknown2k1]

before on cold days it would take awhile to start acting normal then it stopped working.it would raise sudden when i rev high.so i got myself another used gauge cluster, first i just swapped the RPM gauge only and it did not work.so i got the used one and it still doesnt work.both times it just sits there and slowly go the niddle goes over the 8000rmp and maybe 25mins driving it kicks back in and works.got any idea?some kinda sensor?