In ISE, the intellisense times out from time to time for me (5.0.10586.51). The timeout value has been increased to 5 sec in the ISE settings.
A workaround for me is switching the current location from C:\Windows\System32 (default) to a directory that contains less file/ folder. My guess is intellisense scans through the current working directory and having it set to a location with less file count would help.
Maybe this will help .