Diagnosis
The fan is likely overheating due to a clogged air intake or a failing motor thermal protector. It enters a safety cooldown mode.
How to Fix It
First, ensure the rear intake grill and the front output grill are completely unobstructed. Move the fan so it has at least 2 feet of clearance on all sides, especially the back. Clean out any dust inside (see cleaning fix).
If That Didn't Work
The motor itself may be failing and overheating. If it's under warranty, contact Vornado. If not, the motor assembly is the core component and would need replacement.
Heads Up
Running the fan on its highest speed (4) continuously for many hours, especially in a hot room or in direct sunlight, can trigger this overheat protection more quickly.