murts,
Which version of OS2007 did you run before you switched to OS2008? It could be that you weren't having problems as long as you were running an OS version without certain access point power saving features.. and your router isn't power-saving friendly. If that's the case, look up the power saving configuration options in the control panel (I can't be more precise than that because I don't run OS2008 myself), and change the settings and see if it makes a difference.