quite a few of us on the board have m104 engines that behave like yours and some have accepted that it is normal. my own pet theory is that my rad is not as free-flowing as i would want it to be, what with 9 years on the clock and the PO using who-knows-what type of coolant changed at unknown intervals. if you're going to pull the rad, why don't you look into dipping it in an acid bath? i don't really know how feasible this is with the plastic components. i had it done once on a saab's rad some years back - took a couple of hours. they also rodded the tubes but unfortunately found that a few of them were shut beyond repair so that there was little or no discernable effect. i don't believe any amount of backflushing and even citric acid will get rid of the kind of buildup i'm talking about. in any case, keep us posted...
BTW, went through some nasty NYC traffic yesterday, ambient temps 65F. temps went no higher than 95 after cruise - stop and go sequence. this is with my current fix of using an 83C thermostat. with the 87C thermostat i don't doubt it would have behaved like yours!
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"