Unfortunatly once a viscous fan has had its day its a replacement one im afraid. The actual viscous has probably dried up and isnt operating the clutch anymore. There are a couple of things you can do.
1) replace the thermostat, this is always good practise and they are only a few £/$ or whatever to buy.
2) Get a replacement fan
3) Scrap the viscous fan altogether and fit an electric one, they are much more efficient, and dont drain valuable engine BHP when they are running.
The fan you have got is easy enough to test. The fan should come on about 100 degress on your temp gauge. at about 100 degrees, rev the engine, you will be able to hear a rush of air as the fan spins round with the speed of the engine rather than freewheeling as it normally would be. Let the temp creep to about 105-110, if by then the fan hasnt kicked in, then thats your problem, and a replacement is needed. I would never let the temp go over 110 if you can help it.
I note you say that on crusing the temp is fine, so i reckon its the fan thats at fault.