High RAM usage can be a common cause of this, leading to "Lost Connection" messages when trying to open a document, since the document itself either takes too long to load into memory (since the QlikView server will still try to do so, clearing out cached data, and Windows will begin swapping memory to disk), or because the QlikView server is simply too busy to handle any more requests.
Monitoring memory usage and seeing a pattern in high memory allocation when you experience the problem should be fairly simple to do by setting up Windows performance monitoring.
You could also check to see if you can find the QlikView Server Event log logging Working Set Limit alerts. You'd look for something like this (kb 000048671).
Though in a situation like this I would also expect the Desktop Client (open in server..) to be reasonably slow, so maybe we are looking at connection problems between the WebServer and the QlikView server. Since, if you use the Desktop, you bypass that.
Here's (kb 000042484) a common adjustment that can be made in case we are looking at a timeout. It's basically a way to adjust how long the webserver component (and in extension AJAX, our full browser client) waits for the QVS to respond.
Another possibility could be a corrupted TicketData.pgo file (kb 000003857).
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