Hard to tell being I'm not there with ya. But have you looked at the SSRS logs to see if you are getting any messages that might give a clue to your situation? Wouldn't hurt to look at the Events View in Administrative Tools at the logs. To see if there's any errors being thrown when the problem does occur. Look in the Application log. See if there are any messages saying something about the SSRS service not able to connect to ReportServer. Look in the Security log. See if there are any Failure Audits and what for. I've even gone as far as looking at the IIS logs. See if there are any 401.1, 401.2. This might go hand in hand with the Failure Audits in the Security log. If you are in a domain sometimes you could be having intermittent Active Directory issues causing authentic ion to fail. Just a thought but something to check and rule out.

Wouldn't hurt to try to ping the Reporting Service URL and IP, also do try tracert to the IP and/or Reporting Service URL. To discount network issues.

Might need some more info on your setup to help more. Like what version of SSRS is it? Is it a standalone server or web farmed? Is it the only application running on the server or are there other web sites, applications on the server? For example, does it share the same server as SQL Server or is SQL on another server?

See you might be getting intermittent results because there might be a problem of getting response from SQL server. It has to query the RS catalog to get data, resources, etc ... in order to build the Report Manger.

Another thing to think about is how heavy is you SQL Server being used that has the RS catalogs on them. For example: If you have SQL Server on a single server as well as Analysis Services on there also while someone is loading/processing a cube. Could be running out of memory and resources which cause SSRS to lag/fail in response.

Things you could try is recycle IIS. If that doesn't help there's the old tried and true solution to windows -- reboot the server. How long has it been since the last time the server has been rebooted? Disk clean? Drives defragged? Not saying this will fix it, but if it’s been awhile it wouldn't hurt. Not sure the extent of your authority or permissions. Guess that all depends on some of the questions above, like is it a standalone service or is it shared.

If you have SSRS in a web farmed solution you could shut down services on one server and test the issue to see if it's related to one particular server. If standalone, have you tried to hit it the Report Manger on the server to see if you get the same results? Otherwise, does this problem occur from client/user workstation only?

Have you ever used SQL Profiler before? Might have to do a test with someone hitting SSRS Web Report Manger while you or someone runs SQL Profiler to see if by chance that your SQL Server isn’t' responding from time to time. You would connect to your SQL Server that host the RS Catalogs and see if you don't get a request from SSRS and a response from SQL Server. The case you do is run one when it works and save it. Run one again until you get a failure and save it. See if you can see a difference.

I've ran out of suggestions to make as a way to trouble shoot the issue for now.