Suburban runs hot over 45mph

2004 Suburban, 135k miles, is suddenly
running hot at 235 (doesn’t ever exceed that
temp per the gauge) when traveling above 45
mph. When idle or regular city driving temp is
normal and steady at 210. Fan working
properly, AC cold, heat hot, coolant level ok. I
see no external leaks, oil is clean, no signs of
head gasket issue. Coolant is 2 years old.
Engine power is good, no rough idle. Thinking
of changing thermostat as it’s easy and cheap
fix. Next would be the sensor. Any other
ideas?