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See its not about wifi its about the mobile network and no its not because of the signal strength,...
i too have the same problem even i switch off the stamina mode... Its a bug in your software pls rectify in your next 4.3 update at least..
Okay about stamina mode,have you check out which apps that might be allowed to run in background?
Go to Settings->Power Management->Click Stamina Mode and check what apps are excluded during standby in Stamina Mode.