Hi,
You can profile builds with Unity pro, what is the targeted platforms?
so, if you completly remove any global variable network synchronization, you don't get any memory leak?
This is a tricky thing to debug because it could be that the synchronisation itself is the problem but what happens prior or after.
I would do the following: switch off this synchronization and implement a manual ovveride of this value ( like a slider or a joystick input), and feed from the exact same palce the value as if it was sysnchronized, if it doesn't crash anymore then indeed it's the sync, else you'll need to debug further.
It's very rare when a build behaves differently then within the editor, are you sure you don't see any suspicious behavior within the profiler inside Unity? run the server in unity and client publish, and then do the opposite, so you can witness both sides within Unity profiler.