On the local server I use LiteSpeed/2.0RC2 with php5.0.1 neither zend nor mmcache not installed. It gives 503 error last time I run phpmyadmin. there is nothing on the lsws/logs/stderr.log and lsws/logs/error.log only startup logs are there. now I changed the configuration of php to log errors in a textfile, when it happens again I will make you know if it is from php or not. I have not looked at the other server yet but I am using a commercial script on it. It has the default installation LiteSpeed/2.0RC4 with php4.3.10 and mmcache on it. from the statistics it gives %60 percent of its hits 503 errors. As you said I would turn off mmcache optimizaton and make you know then.
On the other hand if the problem is occurs because of php can't it be handled by litespeed and show the error on the page like apache, this would be a big problem. Most of the people debug php with the errors it gaves to he webpage.