Do the V6 engines have the same fan speed resistor issues as the TS? If so, I'd put my money on one of the following two:
* Fan speed resistor is corroded, preventing the low speed fan from kicking in. As you look into the engine bay, the top-right corner of the radiator fan will have an electrical connection going into it. Whip the connection off and remove the resistor (attached to the fan body with 2 Phillips screws) to take a look. Take a look at the EBSpares website if you need to see a picture of what it should look like.
* Radiator needs replacing.
In my case, it was both. It sounds like the latter, but the resistor is a quick and easy check to do before you get your wallet out. My temp got close to 110 in heavy traffic, cooled only by a duff radiator and the high speed fan. Now that both things have been replaced it sits at a steady 90, regardless of outside temperature or traffic.
NB: looks can be deceptive, my radi was shot to bits (about as bad as it could get without leaking) and yet I could only see a small amount of corrosion when it was in the engine bay.