Maybe bad head gasket. I saw the same thing on my 1992 Plymouth Acclaim (remanuf engine installed in 199
in 2000. Many times the temp gauge shot up close to the red line before it quickly went down to normal temp. I changed the thermostat from 192 to 180 F, still the same thing. The radiator was flushed a couple of times, still the same thing. I took it to several machenics, and they could not found anything wrong (and did not check the head gasket problem), and said that the guage or sensor was not right. About less than 1 year later, I noticed the head gasket leak (the engine oil was kind of watery; water found inside the oil cap). The head gasket was gone, which cost me $800 to have it replaced. After the head gasket job, the temp gauge worked perfectly. Because I did not discover the head gasket problem early enough, the remanuf engine'* 3-year warranty expired bafore the head gasket was found bad. FYI, the engine was made by Bond(*?). It was ridiculous that the head gasket only lasted 2 years.