kia mentor electrical and overheating problems?

my 1996 kia mantor glx se has recently started idling very high.When engine starts it idles about 1200 revs, but after a couple of minutes it goes up to 2500 revs. does anyone know whats going on? also about the same time as the idle problem the temperature gauge keeps shooting up and down, from where it should be then all of a sudden it will shoot up to the red line then down again. it is happening to quickly for the engine to have actually overheated then cooled down. can anyone help me?