my 93 d350 overheats at highway speeds of 90 to 100 mph egts are at 950 to 1000 any ideas or is this the nature of the beast

 

How old is the radiator? It shouldn't run hot even at those speeds unless you have a big ol' slide in camper on the back.

 

factory.... nothing blocking it either

 

I suspect it's pretty cruddy inside, 15 years is an awful long time for a radiator.

 

rad been flushed.. cores look soild.. is there any after market rad

 

Is that empty? Try a real temp gauge..My factory gauge is a pos and it had me fooled for a while when I first got the truck ...Does your water pump weep out the bottom when it gets hot? Thats a pretty good sign that the pumps no longer efficient..i spent the money on a new rad when all I needed was a pump/tstat...much cheaper..
Is it getting hot enough to boil over?..If the fan spins by hand when it gets hot you could probably use a new one of those too..good luck,

 

I had a truck that was doing the same thing, it ended up being because all the fins on the back of the radiator down by the bottom where rusted out or missing. Everything looked good from the front and inside the cap, the bad area was out of sight. Might be worth checking.
Or you could do the speed limit.

 

2nd good gauge before you throw a lot of $$$$ at it.