A World Without Her by Robert P. Fitton

Colorado, 1539 - Ricardo and Martin stumble upon an abandoned alien mountain outpost called Cibola. The Entities inside allow Ricardo to jump at will into many worlds. Just before the millennium Ricardo rips Peter Sturgis from Jeannie and his children, and inserts him ahead in time, where Jeannie is a Hollywood star, Jean Carlisle, married to Ricardo. Peter pursues Jeannie but faces his ultimate showdown when Ricardo orders Jeannie's death