I'm having a similar issue. The Object Calculation Time Limit: is set to 60 seconds. I increased this to 600 but the "Lost connection to server", "reconnecting" message still occurred after about 1 minute.
Observing the memory I can see that the report continued to generate in the background. By re-selecting the selections I was able to return the report to screen when in had finished.
Any more suggestions as to the cause / solution.
Check a couple settings:
1. QMC/System/Setup/QVS - Make sure there is no value for "Maximum Inactive Session Time", "Possible Session Timeout" and "Max Total Session Time"
2. Open the task of the dashboard in question - Server/Performance and make sure "Maximum Inactive Session Time" has no value and uncheck "Enable Audit Logging"
Kick off a reload of the task and try again.
Hope that helps,