One of my teams had problems last week with their VEXNet connection being interrupted during matches. They changed to a new cortex, a new remote control, different wireless keys, and fresh batteries in the remote and 9v backup. They practiced all week on our field using the competition switch with no problems. They went to a match today and had wireless problems again. All firmware etc is up to date and we are running the latest RobotC on a Windows 7 computer if that makes a difference. I keep thinking there has to be some hardware problem somewhere but I don’t know where to look now. This is very frustrating! Has anyone else had this problem, and how did you fix it?
What symptom are you seeing that makes you think it’s a VEXnet issue? Do you have a 9-volt backup battery connected? Is the VEXnet key on the robot buried inside a metal box? Are you using white keys or black keys? What are the lights on the joystick and robot doing during the problem?
Who still has the black keys, all ours broke about 2-3 years ago.
Sometimes wifi will interfere with VexNet because they both use 2.4Ghz. You could try turning all of your phones to airplane mode, we always do it at comps. Additionally you could try a USB extension cord, we bought one at worlds and it helped a lot to have the key raise a bit.
could this also be the notorious broken competition port on the remotes?
They need to change that seriously we would have a lot less problems.
What symptoms were caused by that?
We had something similar last competition. Our robot consistently disconnected at the beginning of driver control on field 1 but had no problems on field 2 or when not connected to a field. We tested with our own competition switch with no issues. Ended up costing us the finals match. 
We had a similar probable in the semifinals of our last competition. The second our controller was plugged into the port on 1 specific field,the Vexnet disconnected
Good question, I have had this happen in previous tournaments, could it be that the control switches that you connect to at a match are bad? or is it the controller or cortex? This problem has cost us matches before.
In this case, this won’t help, but to anyone else having this problem, we have not had this problem since switching our cortex. It’s a fairly obvious solution, but if you haven’t tried it yet it may fix the problem.