I have the same intermittent speedometer problem on my 1999 Subaru Outback...179000kms.....my cruise control works fine, I am told that rules out the sensors, they are working fine....or the cruise control would not work.
I have followed all the advice on here, replaced cluster......, cleaned and tighten screws , added ground wires....checked circuit board and connections....... added the wire to bridge the trace...resolded connections....and so on.....
my speedometer is still intermittent...
I have found that I when the heat is on mine will start to work , may work for a day or 2 Min's ....but does seem to be heat related....which brings be back to poor connection...
I am completely out of ideas....any suggestions?