This seems to occur when icons blink on the Windows task bar or in the task tray. It also may occur when Windows Update and other programs pop up a notification window while you're playing OFP.
If your graphics card supports Hardware Transform and Lighting (HW T&L), run the OFP preferences program and select an appropriate HW T&L entry in the DISPLAY TYPE drop-down list.
If that's not applicable or doesn't help, try pressing ALT+TAB to go out and come back to OFP.
(With thanks to squadmate Uziyahu for pointing this issue out to me).