speedometer has a mind of its own

my speedometer is all messed up on my 2004 Buick LeSabre.  I've already
been down the km change road and that is not the problem.  Some days it will
work just fine.  Other days it will stay at 0, go below 0, or act like it is doing
random numbers.  Once I saw the needle go bezerk (back/forth rapidly).  Next
time I drove it, no problems.  But it still happens at random.