gps problem on 2005 pacifica

i have a 2005 pacifica touring with gps. the gps system has been a little inacurate, it would shows me routes that I am not on and it take a while sometime to re-route a new route. when I make a turn the arrow continue staigh as I didn''t turn. and it doesn''t help since I can miss my exit by the time it come back to my location.
at first I though it was do to the old software but just purchase the new 2008 dvd for update and its giving me the same problem. I should of spend the money in getting me a portable gps instead if I knew it wouldn''t work.
 
can someone help? is there any setup step I need to do for the gps to be more acurate? or maybe the gps antenna is not getting the reception it should be getting? where is the antenna located anyway. ?
 
thanks for the help.