Unable to delete or modify a menu

Using 3.04 and Atahualpa 3.5.3 http://fishingnet.com/wordpress/about/

All going nicely till I tried to add more items to a second menu. Could be I just made it too big. Tried to delete single items got a 500 - Premature end of script headers: nav-menus.php

It then appeared corrupted (dropdowns still there, but 5 lines of menu items appeared behind this).

Tried to delete the entire menu and got a 500 - Premature end of script headers: nav-menus.php

Have worked around it by putting up a blank third menu and removing the second one for the mo, but still cannot delete the second one one or add anything new to the 3rd one. same error msg.