a quick test for the high speed fans is to disconnect the plug at the blue painted sensor by the thermostat. the fans should go on at high speed. this is also where the cool harness is plugged into, so make sure that they are working first. i have seen my temps get up there in heavy traffic in the dead of winter (3hr NYC traffic jam). the high speed fans did cool it down but the cycle would then start over very quickly. addition of a cool harness immediately prevented temps from ever going over 100C. old thermostats will certainly add to the problem so i would change that and the pressure cap just to eliminate those possibilities.
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"