Ahh good old Gimbal Lock.
Here is a link that explains it well. If your intrested in whats happening.
http://www.anticz.com/eularqua.htm
To fix it is simple, just hold down the alt key and left click and drag in the prespective window this will grab the view and tilt from the mouse's position and free the gimbal lock.
Trying useing the alt left-click drag next time you want to view your object in the prespective window. Useing this gives you full and free rotation of your object which makes it much easyer for, looking at the bottom or rotateing it quickely to see any angle you wish.