I drove my '90 Civic EX sedan 1.6L (180k miles, 5spd) on a 65 mile trip
last week (averaging 75mph, 3.5-4k rpm, no AC, ambient temp 85) and
about 5 mins after I pulled off the highway, the temperature gauge
slowly climbed to 2/3, up from its normal position at just under 1/2.
Once I parked, I heard gurgling coming from the coolant overflow
bottle. After about 4 hrs, I opened the radiator cap to check coolant
level, it was still topped off even though about a cup leaked out of
the top of the OF bottle. OF bottle was empty, however. I
trepidatiously drove home, this time keeping my speed at 60mph or
below, and had no problems.

This was the first time since buying the car last fall I drove it at
this speed for a prolonged time, have driven several 1/2 hr trips at
less than 60mph since and no problem at all.

There's no oil or HCs in the coolant, no coolant in oil, so I'm
relatively confident there's not a head gasket problem, but who knows?
This car's so old and only cost me $1000, so if that's the case it's
junkyard time anyway. I plan on changing the thermostat, coolant hoses
and rad cap this weekend, and possibly the thermal switch that controls
the fan... don't recall ever hearing the fan run so that might be the
problem right there (anyone know off-hand where this switch is
located?). Perhaps the airflow at 70mph was enough to cool the engine,
but after heavily taxing the engine for over an hour the flow at 30mph
wasn't enough since the fan wasn't working? Maybe a collapsed lower
hose? I changed the timing belt this spring and did water pump change
then too... also, radiator is relatively new (according to previous
owner).

One other problem (maybe related, maybe not): I have noticed an
intermittant squeal coming from the front left wheel that is in time
with wheel rotation, and is more pronounced when I turn left. It's
quiet during right turns, and whenever I apply even light brake
pressure. This noise was very pronounced when it was running hot after
the first half of my trip last week (only after I got off the highway
and was driving for 5 mins at ~30mph on city streets). Possibly coolant
leaking on brake pad/caliper?