I have always found this more stable and exact. I pasted yours into the fireworks demo and it changes way to much (unreadable for me).
'FPS_Counter <> Runs And displays the FPS '-------------------------------------------- SetRotation 0 'Undo rotation SetScale 1,1 'Undo scale FPS_Counter=FPS_Counter+1 If FPS_Counter_time+1000 =< MilliSecs() FPS=FPS_Counter' <- Frames/Sec FPS_Counter=0 FPS_Counter_time=MilliSecs() EndIf DrawText "Current FPS: "+FPS,20,20 '--------------------------------------------This is porbably not optimal, but it gives a accurate FPS for tweaking