Tom,

Not sure where you are located (meaning that I do not know what your local temperatures are at the moment), but if your temp is running 100-120 this time of year (winter) you do have a problem.

1. It sounds like you have opened up the system to at least replace the thermostat lately. Have you checked to make sure that all the air is out of your system (after replacing the thermostat)? Your temp sensor (on top of head toward back - and usually a high point of the system - can be unscrewed to release any air trapped in system. If coolant flows out, you are good; if not, add coolant until it does. Squeeze top radiator hose to break trapped air free ... do all this with system cool, BTW.

2. Have you checked your serpentine belt to make sure it is tight? - should be able to push down on it and move it no more than @ 1/2 inch. If more play, need to tighten - search for "belt tensioner" on this forum for adjustment instructions.

3. You say no leaking at present, but have you looked around in engine bay to make sure no residue from dried coolant (white spots/splotches w/MB anti-freeze) seen anywhere. You could have a small air leak starting, prior to leak showing up. Drive for a spell to get engine up to normal operating temp, stop, pop hood, listen for hissing/fizzling sounds; if any sounds, trace to origin. Check around and under coolant fill tank when doing this also.

4. Does your car cool back down to 85-90 when you leave traffic and get up to speed? If yes, fan clutch is a suspect and radiator a less likely culprit.

5. Have you tried turning your heater to full high with fan on full high and seen system cool down? If yes, radiator is more of a suspect - could be blocked and require either cleaning or replacement. Before doing either you would want to check for cool spots on radiator (where blockage is). There is a tool (search for "infrared" on this forum, I think) for reading temps on differnt parts of rad. Replacing rad not that bad of a chore - go with Behr if you have to go this far.

(all this from 103 engine, which I believe is plumbed pretty much same as yours.)

Possible solutions arranged from least to most expensive.

Hope this helps.