Replace the t-stat if it's not recent - use OE/OEM only (Behr, Wahler). It's located where the lower radiator hose connects to the engine. Unscrew the round sensor with one wire attached directly in front of the housing, this will make it MUCH easier to R&R the t-stat. (Trust me.)
About the overheating, assuming the t-stat is good, rad cap is good, there's no leaks and the system holds pressure, and the radiator & condenser fins are clean... you either have a bad fan clutch, internally corroded radiator, or both. The clutch is more likely especially if it's original. You can refill it with silicone if the bearing is good (search for details).
The electric fan should trigger at 105°C... it's controlled by the 3-prong switch shown in the photo below. With the engine running, unplug the 2-pole connector and short the two female sockets together... the fan should turn on. If so, replace the switch. If it doesn't, figure out why (and I'd still replace the switch on principle... I've yet to see one that still works after 15 years.)
