Unable to server this request

[eluser]Unknown[/eluser]
It has been 2 days since I'm messing with this issue.
When a controller takes more than 15 seconds it gives me this error

http://nsn.stabletransit.com/unavailable-ord1.png
Site temporarily unavailable.
Connection timed out - please try again.

If it takes less than 15 seconds in processing. then it works fine.
I'm processing images in zip files and checkout CC etc

Please help. see attached