Milano Cooling Problem

["Kevin Smith" <kevin.smith12@domain.elided>]

Actually it runs cool.
My 87 Milano 2.5 with almost 150K miles on it is loosing coolant out of the
overflow container cap.  The car always runs between 175 to 180 degrees with
the fan actuation being the only difference.  I have checked everywhere for
leaks and can find none.  I can let the car idle in my driveway until the cows
come home on a 95 degree day and loose no coolant.  However, when I take it
out on the freeway (70 to 80 mph) for a few miles the coolant light on the ARC
panel comes on.  Sure enough, coolant lost through the cap (thank goodness for
the ARC panel and the sending unit).
I have replaced the overflow bottle and tried two new caps to now avail.  I
just put in a new thermostat to no avail.
Now for the heavy stuff.  I bought this car about 35K miles ago and it has
always had a dirty overflow bottle (looks like oil around the edge but not
frothy in the fluid).  The car doesn't use much oil, never smokes, and drives
great.  The oil appears to be free of any water.
Everyone I talk to suggests that I have head gaskets in my future (I'm in
denial).  I'm hearing that combustion pressure is getting into the coolant and
blowing so much pressure that it comes out the cap at speed.
How can a car that runs so well, has no water in the oil, blows no smoke
really need head gaskets?
Any opinions from the digest hold more water (pun intended) that the ones that
I have gotten so far.
Thanks in advance!

Kevin Smith
86 Spider
87 Milano
2000 S2000
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