Engine temp guage

engine, temp, guage, problem, ahve, promlem, wrong, cold, read, turbo, running, tempature, bottom, bouncing, lucky

so my engine tempature guage im my 91 mr2 turbo keeps bouncing up and down. it seems to read the running temp like it should (if im lucky) sometimes it goes down to the bottom like the engine is off or cold. does anyone have or had this problem and if you ahve had this promlem how would i fix it or whats wrong with it