Why does this happen?

Well, it's finally the time I ask this question here :(I play FS in windowed mode, to get higher FPS (at least that works for me). when selecting the resolution from the Display Properties (Hardware section) I select the one I'm running in my desktop. BUT...I found out, after a while I get lower FPS...and if I change the resolution to another value, FPS seem to go up by 10 or so...after a while they return to the "low" mode though...I could be changing the resolution every two mintues and get high FPs all the time, but there's gotta be another way...So the question is, why do I get higher FPS right after changing the Resolution if I'm running in windowed mode? I know it sounds wierd, but maybe it's something we can take advantage of....Some infoSystem = P4 1.5, 256 RAM, GF2, Win XP