Overheating

I just got myself a Jimny which was running fine, no problems until I drove it on a moter way? the temperature is fine in traffic or around town but as soon as I try and go over 55 or up a long hill it starts to overheat, if I keep driving at 55 or under or even let it tickover it will come back down.If it was the thermostat wouldn't it over heat in traffic?

brian smc had a similar problem recently on a trip to dunoon
the viscous fan was`nt doin its job properly
so we came up wie a quick bodge solution
we cable tied the fan to the alloy coupler an hey presto fan now keeps it cool try it an let us no the result
you will hear it if it works makes it sound like a v6 lol

cable tie to the pulley its self you mean,? is it of so i will try
also someone mantioned timing could be out?