Rapid fluctuations of the temp. gauge means there is air in the system. This is because of hot pockets of air passing over the sensor. It takes longer than 10 sec. to over heat an engine, but like it has been stated, take it somewhere and have the system pressure checked, this will help find the leak. And jsut for mainatance it would be a good idea to change the LIM if it has not been done in the cars histoy.
Also, bad water pump bearing will make noise and leak, coould be the source of most of your problems.