I think this Stack Overflow question can shed some light onto your issue:
This one also has some good information:
It looks like you might be dealing with a threading issue. One thing I would recommend is putting the NetworkStream declaration inside the Tick method. Also, since you are working with bitmaps and other heavy items, I would recommend turning off the timer at the start of the tick method and then turning it back on after it finishes. That will mess with how often your event fires, but that should prevent a lot of problems. If that isn't viable, you might need to look at using async and a task for this.