In admin console "Web publishing" > "General settings" what do you see "maximum number of connection for custom web" ?
What do you see in FMserver log?
Is there any proxy server between FMS and PHP site?
How about changing URL to
<<your URL>> & "&" & Get(UUID)
The issue was resolved after we moved the web direct connect to a separate server that had no other apps installed.
Because we wanted to have our primary server local, we decided to create a separate table and host it on Triple8.net Filemaker Hosting solutions. It worked great.
FM Support thought that some apps on our main server were interfering with the router communications, which must have been true.