well the readings are wrong, 198C is a garanteed dead card..
It might be the sensor, or it may be sensor/software driver problem or clash.
You could maybe try cleaning out AMD driver and reinstalling or trying a dfferent driver.
What does the catalyst/overdrive thingy say your temp is?
You could also try HwInfo to see what temp it reads.
PS broken temperature sensor won't stop your card working but it might be a good idea to set your fan speed manually to some "safe" speed to keep it cool at full load.