The viscous coupler could produce most of the symptoms that you describe, but I am not sure that it would produce a clunking noise. That is the one part that has me stumped, but I do think that the diagnosis of a bad viscous coupler certainly has some merit.
I would suggest that you pre-arrange with your mechanic that, if the installation of a new coupler doesn’t fix the problem, he would remove it and put the old one back in–and only charge you for labor.