Cadillac: 1997 DeVille over heats at low rpms, blown head gasket, head gaskets

Question
My mother-in-law owns a 1997 DeVille with a slightly blown head gasket.  Trace amounts of white vapor out of the exhaust and their mechanic said they needed a new head gasket.

The car has been constantly overheating.  She needed a car to drive but they can't afford a new one and don't want to put any more money in the DeVille.

I went up to see them last weekend and took the thermostat out of the car.  When I ran the car I noticed it would still slowly overheat at low RPMs, but would not at slightly higher ones.  So if I drove the car in 2nd and 3rd gear it was fine but if I drove around in drive or idled in neutral the temp would climb up to the 250's and soon the car would start to overheat.

Why is it doing this? and is there a better cheap way to get some more life out of the car than leaving the thermostat out?

Answer
NEVER EVER leave the thermostat OUT of a fuel injected engine as it totally screws up all the temp and other sensors and causes severe drop in fuel mileage and in some cases transmission shift problems.

Either replace the head gaskets or trade the car.