I loved The Thorn Birds and some of Colleen McCullough's Roman epics, so I was eager to pick up her latest, a serial-killer thriller set at a very Yale-like Ivy League college in Holloman, Conn. When the first body part is found in a university lab's refrigerator, Lieut. Carmine Delmonico is put on the case. Despite his careful casework, the killer eludes him, leaving behind so few clues that Delmonico begins to think of him as ''the Ghost.'' Yet for all McCullough's skills savvy plotting, great dialogue On, Off just doesn't ring true, and that's because Delmonico is too familiar, even bland, to work as the main character. There's no question: McCullough can write. But maybe she should stick to historical fiction.