Seems unusual, a blown head gasket that is. Failed thermostats are much more common, mine failed in the "open" position, sometimes they fail closed if somebody is really unlucky.

Fortunately there are piles of used 5-cylinder Volvo motors around and they're (relatively) cheap. It seems people wreck the cars much more often than they melt the engines. Try the used part finder "copartfinder.com" and the Volvo recyclers such as Erie Vovo. I don't think that a '98 motor will work in a '99 because they switched to variable valve timing with the '99s. I think a '00 might, though. Do check with somebody who knows, like an indy Volvo shop. I don't think you want to know the price of a factory Volvo rebuild.

FWIW, my daughter has brought her car home with all the red lights on the dash board aglow (thank God for cast-iron engine blocks!) and has driven 2 miles on a flat tire. And she is NOT a dingbat! She is currently a student at a top nationally-ranked college, and received her High School's "Technology Student of the Year" (aka the Auto Shop award) among several other awards at graduation. So I wonder if this problem might be genetic and sex-linked, a tendency to focus on a single task (like driving) and filter out peripheral input like warning lights, chimes and flat tires.

-Punxsutawney Phil