Most everyone on Yahoo Mail Beta is having similar problems. I am too during the day! I have cleared the cache and history, booted into SafeMode with networking, cleared all temp files and still cannot fix the problem.
This is a definite Yahoo issue and they are not providing any info as to why so many are having the problem.
I suggest switching back to the original version. You cannot change back to the original w/o accessing beta first. It works at night. Log in then to use the Switch Back link.
There is a site you can access to check your Beta mail if you keep getting Page Not Displayed or a blank screen. It is crued and text only, but you at least you can check your mail. Here is the link:
Yahoo! Front Page
Oh yes, and for all of the people that want to call Yahoo support, it does cost $29.95 to speak to a live person!