iDevices disappear from iTunes

I have searched the Communities and not really found a solution.

For some time now I have had an issue where my iPad and iPhone will randomly disappear from iTunes. Generally some combination of restarting iTunes or the device will solve the problem. Sometimes plugging the device into power works (it is very rare that the battery is low, I just charge often). Often the device will initially show up )but sometimes not) and when I try and sync (over WiFi) it will then disappear, with a message saying that "xxx can not be found). If it doesn't work from iTunes then it doesn't work initialising the sync from the device either. I'm not convinced my devices are automatically syncing when plugged in, though I have those settings turned on.

My iMac is always on and iTunes always running. My WiFi network seems to be stable and I have no issues with internet connectivity over it, nor with streaming to my AppleTV's. I keep all my software up to date, and this problem has been occuring over several generations of OS X, iOS and iTunes.

Another thread suggested it may be something to do with iTunes .plist files but didn't specify what needed to be alterned. Any help would be appreciated.