I'm experiencing very low perfomance on my pc when playing these sandboxes; either modern city builder or D.'s sandbox.
Things appear and disappear like something is going seriously wrong, even if I set all settings to low I still have poor perfomance.
Please, tell me how to fix it. Oh, and gimme some command for fullscreen mode like the normal sandbox, not stretched window, real fullscreen

My sys specs:
CPU: intel dual-core 2.0GHz
RAM: 4GB DDR2 533MHz
GPU: nVidia 9 Series (9400GT, 512MB)
System: MS Windows XP 32-bit SP3 5.1.2600
I highly doubt it is because of WinXP or my cpu or ram, as I have GTA EFLC and there is no lag with that game.