Known limitations on error_log?

I am using php4 on an Apache server on W2K and am using the error_log function to write events to a file (log type 3). Under a load of six or more concurrent executions of the script that is writing to the log file, I get incomplete entries in the log file. The remainder of the script executes (downstream functionality processes without a hitch), however the log file does not reflect this activity. Is there an issue with concurrent write access to the log file? Do I need to setup multiple log files (not desirable from an analysis standpoint) in order to alleviate this concurrent access issue? Any help would be greatly appreciated. I rely on these log files for accurate reporting of what is being processed by whom and when! Thanks, M. Brian Dunson