COLD FIRE by Dean R. Koontz (G. P. Putnam's Sons: $21.95; 384 pp.) . Dean R. Koontz's imagination is not only as big as the Ritz, it also is as wild as an unbroken stallion. While some Koontz fans may find "Cold Fire" relatively tame and low-key after the truly bizarre "The Bad Place," this latest offering is fully as gripping in its own way. What is it that prompts Jim Ironheart, an otherwise mild-mannered schoolteacher, to fly from his home in southern California to Portland, Ore., just in time to save a young boy from a drunk driver? What drew him to that particular street adjoining that particular school at that particular time? The rescue piques the curiosity of reporter Holly Thorne because it appears he has been involved in six such Perils-of-Pauline rescues in six far-flung cities in the past three months. Koontz has become adept at holding out the promise that something very creepy is lurking around the next corner, and sure enough, it is.