After I cruise for 15 min, the temp will usually stayed at about 80-85C, but if stuck in traffic, the temp will go as high as 100-110C(A/C off, outside temp at 25C), is it normal or do I have a problem??

Regard..

'92 400SE

I don't have as much experience with the W140's, but I know that sounds very normal for a W124 4.2 V8. If it stays above 100C while cruising at highway speeds-worry, if the temp gets above 110-115 while sitting-worry, what you describe sounds normal to me. While sitting the electric fans should kick on when it gets above 100C or thereabouts. A/C usage will tend to raise engine temp, but not in such a way that you should be concerned. From your description, it sounds like things are as they should be.

Hope this helps...Lee

Your vehicle has a multi-speed auxhilary fan setup that changes speed according to coolant temperature. If you are concerned you could have the lower speed controls for the auxhilary fan checked to make sure all speeds are working properly.
Listed are the fan speed stages and the coolant temperature for switching
1ST STAGE - 100 C - ON, 95 C - OFF
2ND STAGE - 107 C - ON, 100 C - OFF
3RD STAGE - 115 C - ON, 107 C - OFF