I have a '99 Honda CR-V EX. It's been a great vehicle for us but today
I noticed a problem with it (My wife usually drives this vehicle so
it's possible it's been happening for a while, I don't know):

When driving at low speeds (ie, < 35mpg?) the temp sensor is right at
the normal, middle position reading.

When at idle, it is slightly (very slightly) higher than normal. So
small a difference that by itself I wouldn't be concerned.

When at higher speeds (> 35mpg), the temperature gauge drops rapidly
down to the C (cold marking, ie, the lowest point).

When acceleration ends like coming to a stop light, the temp reading
goes back up.

I immediately took the car to get the oil changed and the coolant
flushed and filled. Still happening.

I did some searching on the internet and couldn't find anything other
than vague references to the following possible causes:

water pump bad
thermostat
a temp sensor
a blown head gasket (oh I hope not!)

Any ideas? I plan on dropping it off at the dealership tomorrow. It's
not under warranty.