I've got to add my .03 cents here (inflation). Last year I too had cooling problems that were kicking my butt. The viscous fan seemed fine, no noticeable fore and aft play, resistance while spinning it, overall I dismissed it as being a problem. UNTIL........
I got the engine to operating temperature, shut it off and started my usual looking at everything and going into deep thought (I'm sure everyone has done this at least once while staring at your engine).
Anyhow, I reached down and grabbed the fan and moved it fore and aft, you wouldn't believe how much play I had in the clutch (after the engine was hot). It looked fine and felt fine when cold, but when it was hot is when all of the play showed up and I was able to pinpoint it as being the problem.
Try that and see what you've got if you haven't already.