Overheating

We are at a loss with my Yukon! We have replaced
water pump, thermostat, radiator, coolant temp
sensor. There are no leaks in hoses. The oil is not
milky, so we don’t think it’s a head (besides the
overheating, it runs like a top). A/C and heater is
working fine, doesn’t get hotter or cooler when
on/off. We have purged any air out of the system.
We have hooked the computer up and it’s not
throwing any codes. At idle it stays between 210-
220. Take it down the road and does fine for maybe
10-15 miles and starts running up to around 230-
240. Of course, it would run all the way up to 260+,
but it’s already happened about 3 times so if it gets
around 240-250 we shut it down, don’t want to end
up cracking a head! At a dead end with what else to
check, ANY help would be appreciated!