Raising limits will not help you situation. The most common reason for the 503 errors if your php processes crashing or taking so long to process they might as be considered zombies.
You need to monitor stderr.log, error.log, or php.err for any php related errors. In addition, we might be able to help you if you provide the actual code to the script causing the problem.
Also check for any core.XXXX files within your php file directory or within /tmp/lshttpd/.