I drive a Plymouth Sundance 2.5. I'm having several issues with my
temperature gauge. My car doesn't seem to actually overheat, but the
temperature gauge reads on the high side sometimes. This seems to only
happen in hotter weather when cruising the freeway at higher speeds
(and also when running the A/C as well). Basically, when I'm just
driving normally around town and stuff on smaller highways, my temp
gauge usually stays at about 180-185 degrees (I have a 180 degree
thermostat) when going down the road and fan comes on at 211 degrees
when stopped.

Now when I drive at higher speeds on the highway (like 80 MPH or so)
for a while, my temperature gauge will rise to about 200 degrees (1/2
on gauge) and peak at about 210 degrees (a little over halfway) when
cruising. This is a little more obvious with the A/C running. And when
I stop at red lights or sit in a parking lot (mainly after extended
highway cruises), the engine heats up to what appears to be 225 degrees
(almost the 3/4 mark) and the fan comes on and cools it back down to
195. When driving around town for a long time (w/o interstate driving),
my temperature gauge will usually stay at around 190 degrees when
driving down the road.

The temperature gauge doesn't do any fast swings, though.

My thermostat is most certainly opening properly. When the temperature
rises over 210 degrees, the upper radiator hose feels hot, so the
thermostat is open. Usually, when the temperature gauge gets to 180
degrees (1/4 mark) when I first start driving, the upper radiator hose
becomes hot. So, the thermostat opens on time. All of the air is bled
out of the system (I drilled a small hole on top of thermostat to
self-bleed the air).

Late last year, I had the head gasket changed, head resurfaced, and a
valve job done. Early this year, I replaced the radiator, hoses, and
radiator cap. I changed the thermostat last week. I replaced the
alternator belt (which drives water pump) several days ago. My coolant
was changed earlier in the year when radiator was installed.

Is this behavior normal? Could the temperature gauge be rising after
extended periods of time primarily because there's extra load on the
engine and the weather is hot (I live in the south)?