Ok, a quick thing you can try is: Find your game install on the HDD (by default its C:\Programs\HTC\Aces High II\) and delete the "Cache" folder - this will cause the game to rebuild the cache on startup... sometimes a corrupted file can cause the problem you describe.
If that doesnt help, were going to need more info to help you troubleshoot - your computer specs and drivers (best would be the top third of a dxdiag report) and ingame vid settings (screen res, texture size, preloading options).
Was there anything you changed since the last time AH ran fine?