John,
ACTIVATE MENU _msysmenu NOWAIT will do what you want if your menu's name is _msysmenu. Was the menu created with teh menu designer? Did you code your own menu definition, if so what is its name? Replace _msysmenu with your menu's name.
BTW, this si not a bug. VFP refreshes the menu when it is not busy with other things. When you set skip fors the menu items will be dimmed the next time the menu is refreshed. The user clicking on the menu forces a refresh, as does issuing an ACTIVATE MENU menuname NOWAIT.