OpManager URL monitoring

Hi. I'm looking to monitor URLs on a number of webservers from OpManager. I'm having mixed success.
Standard HTTP to any web server (TCP 80) is working fine.
We also want to monitor a custom port of 6757 on the same webservers.
HTTP TCP connections are established between the OpManager probe and the webserver on port 6757 on them and an error 403 is returned (as expected and wanted by us). OpManager then reports for one that the port is up and running but on a different one that the service is down (No Response).
I've used the string "<h2>403 - Forbidden: Access is denied.</h2>" as the match content on both example servers; one works, the other doesn't.

Another niggle is that the response time varies between showing 0ms and (average real response of) 16ms.
Also, on the webserver that's reporting that the ports are up and running, the response time is showing as "NA".

Thanks