83 300CD

Ambient temperatures in high 90s to low 100s last week or so. I've made two two-hundred mile round trips in the middle of the afternoon with the AC running, cruising between 70 and 80 mph. After replacing the thermostat (new one boiled and checked out prior to installation) and the temperature sender on the engine block, and of course filling and burping the cooling system appropriately (Zyrex g05 of course), I'm getting the following readings:

Between 70 and 75 mph, 100C
75-80 105C
80+ rising up to around 110C

The fan clutch seems to be working as there is no play, plus I don't think that's the problem because in town, in city driving even with the air conditioner on, the temp settles around 90 and never above 95C. It's just curious to me that its running hotter and hwy speeds than stop and go in town with the same ambient temperature readings. Seems like it's something other than t-stat or even radiator. Would the water pump be involved in a problem of this sort? Or perhaps a blockage somewhere?

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1983 300CDT -- 177K