'99 CR-V Temperature Gauge Normal on Idle, Way Low at higher Speeds

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.

 

==========================

Have you made sure the coolant reservoir is filled to the MAX mark, per
the owner's manual? If not, AIR could be the culprit, preventing the
sensors from registering properly. It takes at least two or three
'trips' to expel air. The engine has to go from hot to cold to allow
coolant to replace the displaced air.

I guess you can pay the dealer for something you can do yourself . . .

'Curly'

 

Well, it turned out to bea bad radiator and also a bad hose that
connects to it. The dealer service manager said it looked like a rock
went through the radiator, and the hose was leaking. $500 down the
drain for the radiator/hose w/labor. Plus I wasted $80 getting the
radiator flushed/filled the other day, thinking that would fix it. Oh
well.

 

go and get your money back. something so obvious should have been
spotted and they should have advised you to replace, not charged you anyway.

 

If it were real mechanics who did the flush/fill I would, but
unfortunately I took it to jiffy lube. I usually never go to those
clowns but it was snowing out and I figured it needed the flush/fill
anyways and there might be a chance that it would fix the problems.

It cost me $500 total for the new radiator and labor at the Honda
dealership.