I'm not a flex guy but you might want to try and increase your parameters in your Geoprocess Service (in Server Manager). I would bump up the number of records to return by 10 fold (just to test and then drop it back down if it does work)
Maximum Number of Records Returned by Server:
I would also bump up the pooling (timeout) numbers (once again just to test)
Timeouts
The maximum time a client can use a service: seconds
The maximum time a client will wait to get a service: seconds
The maximum time an idle instance can be kept running: seconds
If you download fiddler and trace your error you will get a better description of what is happening:
Download Fiddler,
Start Fiddler
Start your Geoprocess task
Wait until it fails
Look at the last few lines of communication between your server and your machine
Select the Inspect Tab
In the right top window select Webforms to see what your machine was requesting
In the bottom right window select Text View to see what the server sent back...here you will find a clear error message
Jay