World on a Wire (1973)

Somewhere in the future there is a computer project called Simulacron one of which is able to simulate a full featured reality, when suddenly project leader Henry Vollmer dies. His successor Dr. Fred Stiller experiences odd phenomena. A good friend, Guenther Lause, disappears in the middle of a conversation and a week later nobody has ever heard of him. And those fits of dizzyness - Stiller cannot believe himself to be fool. There has to be an explanation for all this. Could Simulacron have something to do with it?