Now that you mention it, it's happening only Firefox (although since it's intermittent it's hard to say for sure). It seems to be working OK in Google Chrome.
It only happens in the Page edit of Exhibit Builder, I haven't had any problems with editing Items, nor with editing text in the sections and exhibit metadata screens.
Below is the system info:
Browser: Mozilla/5.0 (Windows NT 6.0; rv:11.0) Gecko/20100101 Firefox/11.0
PHP: 5.2.17 (cgi-fcgi)
OS: Linux 188.8.131.52-grsec-2.1.14-modsign-xeon-64 x86_64
MySQL Server: 5.1.53
MySQL Client: 5.0.51a
Regular: bcmath, calendar, cgi-fcgi, ctype, curl, date, dom, exif, filter, ftp, gd, gettext, hash, iconv, imap, json, libxml, mbstring, mcrypt, mhash, mysql, mysqli, openssl, pcntl, pcre, PDO, pdo_mysql, pdo_sqlite, posix, pspell, Reflection, session, SimpleXML, sockets, SPL, SQLite, standard, tokenizer, xml, xmlreader, xmlwriter, xsl, Zend Optimizer, zlib
Zend: Zend Extension Manager, Zend Optimizer
From Scratch: 1.1
Minimalist Omeka Theme:1.1
Santa Fe: 1.1
Thanks, Roy: 1.4
smithmain: 1.1 (current)