Don, I'm not sure about 300's, but in my 190 D (2.2l-sloooowwww...) I have an electric fan clutch. it ALWAYS turns freely until the fan switch kicks in (around 110C) and then the main fan works until temp goes down to 100C. I had the same problem as yours-after about 1 hr of driving (65-70 mph) the temperature would climb to about 105C while driving. problem went away after I replaced the radiator. Also had a bad rad cap resulting (after radiator replacement) in about 101 C cruising temp. Now, in 80F + weather, the temp never goes above 95C while driving. However, after hard driving when stopped and idling, the temp goes up immediately to about 112C and then comes down after the fan clutch is engaged with the fan switch.
I hope it helps.