Apache2 reverse proxy seems to intermittently freeze?

I would try to use ajp to connect to tomcat from apache.
Please test the configuration using mod_proxy_ajp with Tomcat.
From logs, it looks like the tomcat did not responded to apache's request. It refused the connection.

If you have high traffic, search deeper in your application's logs. It could be a deadlock or maxthreads reached.

HTH

Check that your apache file cache is disabled.

# LoadModule file_cache_module modules/mod_file_cache.so

If that doesn't solve the problem (shouldn't, this doesn't seem a cache problem) try debugging using mod_jk. Download appropiate mod_jk version for your Apache, setup it and set the log level to DEBUG

# Set the jk log level [debug/error/info] 
JkLogLevel          debug

Also, include tons of info in your apache logs

LogFormat "%h %l %u %t \" %m \"%V\" \"%r \" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" %D CustomLogFormat
. . .
CustomLog "|C:/Apache2/bin/rotatelogs.exe C:/Apache2/logs/access_%Y%m%d.log 86400" CustomLogFormat

You should now have an Apache log with microseconds taken to serve the page, and a debug-level log of your AJP 1.3 connection to tomcat (with mod_jk).

With that done, restart apache and try forcing that freezing error. When error happens, check time, and browse logs to see what happened in that instant, how much time did apache spend processing, and so.

It won't solve anything, but most possibly will give you some insight on what is happening.