This one is a real doozy and I'm going to try to condense this as much as
possible.

About three weeks ago, the water pump went out on my '98 Sable.  A couple of
the impellers were broken off.  The temperature climbed a little, but never
got over about 85% of maximum.  I shut her down really fast to avoid engine
damage.  Coolant rolled out of the overflow tank (there is no visible radiator
cap on this car, by the way.)

After replacement, everything was fine for a couple of weeks.  Then, about 23
miles into my 38.5 mile commute to work, after the temperature had remained
steady and normal the whole time, I was going about 70 miles per hour and the
temperature slowly started to climb, but not nearly as fast as it did when the
water pump went out.  I had time to judge what might be happening, but it
continued to slowly climb so I pulled into a restaurant and shut it down and
lost coolant through the overflow tank.

I checked for blockage of airflow, but didn't see any.  I removed the
thermostat before driving it home.  I made it about as far as I'd come on the
way to work, and the temperature climbed again.  I had to pull over about 900
yards from the house.  The mechanics at the shop found a plastic ice bag
between the condenser and radiator where I was unable to see.  I would have
checked for that myself, but I never imagined anything could blow up into that
little space and I would have had to stick my face under the car and look up
in there.  I would have if I could have imagined, but I just didn't.  They
pulled that out and installed the new thermostat I'd purchased, then test
drove it and sent me on my way.  They gave me the bag, too.  It had not been
melted, so there shouldn't be any plastic clogging the fins of the radiator.

On the way to work that same afternoon, the temperature again remained steady
and very normal, then started climbing again, this time about 17 miles into my
trip, just like it did when I'd had the ice bag embarrassment: just slowly
creeping up.  I pulled over again and watched the water roll out of the
overflow tank again.

Now at none of these points did the car ever get into the "danger zone" of
overheating.  At the most, it hit about 85% of the maximum reading on the
gauge.  My dad taught me well.  When it starts overheating, kill it quick!

First, a few key points:

    1. The car has approximately 153,000 miles on it, but has
       been very dependable.

    2. This car has never been one to overheat.  It's always run
       just dead on normal, even in heavy traffic and in the
       hottest part of the day, just right at 45% up the gauge.
       It's never even hinted at even climbing a single hair
       over 45%.  I even tested it one day by letting it idle on
       a hot day at 105º with the air conditioner on for 30
       minutes and the temperature didn't climb at all.

    3. I'll be driving along and the temperature will be normal
       for 20-25 miles, sometimes more, sometimes less.  Then
       suddenly it'll start climbing.  It doesn't go up, then
       down, then up more then back down, then up even more,
       then back down.  It just starts slowly creeping up and
       doesn't stop.

    4. The engine cools down VERY quickly after it starts
       getting too warm.  The mechanic said that once the
       temperature climbed, he turned the key off and let it
       coast for just a bit.  He started it back up and the
       temperature had already returned to normal.  And no, the
       temperature gauge is not malfunctioning.  Indeed, when
       the temperature gauge is up there, there's no mistaking
       the coolant boiling out onto the ground, or the hot,
       steamy cloud rolling out from under the hood.

I got it back to the garage and here's what we know so far:

    1. It's been flushed and reflushed and reflushed again.  The
       mechanics believe the system is clean.

    2. It's been pressure tested and pressure tested again.
       There are no leaks.

    3. The new water pump was replaced with another new water
       pump.

    4. The new thermostat was replaced with another new
       thermostat.

    5. The mechanics say I do not have a blown head gasket.

    6. The mechanics say my radiator is not clogged up.  (The
       radiator that's in there now is a new one I had installed
       back in December of 2003.)

    7. The mechanics say my heater core is not clogged up and
       that the recall work to provide a bypass route around the
       heater core has already been installed.

    8. The mechanics checked to make sure the lower radiator
       hose was not collapsing.

    9. The mechanics say there are no air pockets in the engine.

They're out of ideas.  They're now outsourcing opinions on iATN, which is
absolutely fine with me.  I'm glad they're doing that instead of just telling
me they give up.  But I haven't worked in a week now because I require my car
during working hours.  I'm posting here to absorb all possible ideas from
those who might have run across this mysterious behavior in a car before,
particularly a Ford/Mercury.  It doesn't seem to be to be caused by any common
problem and everybody involved seems to be at a loss.

Any ideas will be appreciated to no end.

Thanks,
Matthew