it's annoying, i'm not sure what rpm i'm driving.
can it be fixed?

Check that both connections (engine and back of rev counter) are fastened properly.
If they are, take the cable of and try gently to pull on of the ends out -> might be broken.
If that's fine as well, prob the spring in the rev counter is broken, can't be fixed afaik.

i just replace to a new cable, but it was dancing befor...

That just leaves the rev counter itself.
You can open the case and have a look: Take it out and use a small screw driver to "unbend" the rim that holds the black metall to the white plastic.
Don't expect you'll be able to fix anything in there, but having a look won't hurt (if you don't slip with the screwdriver and pierce your hand...)