Your problem is not sufficiently defined.
Are you running out of memory?
Are you running out of disk space?
Do you have an intermittant network connection?
"Freezes" is not very helpful.
Can you use <CTRL><ALT><F1> to switch to console?
If so, what does "free" show?
What does "df" show?
From X, <CTRL><ALT><BACKSPACE> should stop X.
"startx" will restart it.