We had a long forum discussion about connectivity issues last season but were never able to reach any conclusions. We came up with several recommendations, including:
Keep Vexnet key away from motors and batteries.
Use a USB extension to get cortex Vexnet key in better line of sight to joystick key and away from motors/batteries.
Keep cell phones away from joystick and avoid wifi hotspots.
Since we were still never able to get a clear diagnosis or solution, I contacted Vex to see if they had any information that would help.
First they offered to replace the key, which I did not think would solve the problem since it occurs with multiple keys.
Then they stated the following.
"We have tested for interference from motors & batteries and found no evidence to support a potential issue. I do know that its best if you mount the Cortex on the side or top of the robot, its not recommended that you mount it in the center of the robot as it will basically be inside a metal cage, which blocks signal. Also connect the key directly into the cortex, dont use an extension cable."
So, basically we have made no progress at solving this problem.
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Thanks for contacting Vex and passing on the information.  My comments on their comments:
- Vex robots are a metal cage,  less so BEST. 
- USB extension affect USB timing,  I know I have several 18" gold plated USB extensions that don't work for this purpose.
- Re "motors & batteries",  Did they say BEST motors?  That may be the biggest difference between Vex and BEST. 
-- Also, Vex EDR motors have been shipping with balun cords for a while now,  to reduce EMI I assume.
-- Vex changed to "turbo" hump case with a higher stack USB mount because their own tests showed that an extra 0.5" away from CPU was helpful.
-- Anecdotal report from a hobbyist:  wrapping foil around wires from motor port to motor was helpful.

Re "Problem with multiple keys" :  How many pairs of key combinations have you tried?  

Have you asked about connection issues on Vexforum?  or asked JPerlman?   I looked briefly but didn't find anything.
Also, Game discussion with Ralsobrook has also pointed out the timed vs connection drop issue, which I have forwarded to game design chairman, and 2019 Game design team.
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Per Vex, "No, the hump is just so user can connect the key more easily. Also the BEST motors should not cause any kind of interference."
We have not monitored which keys are being used at which time but it sounds like we need to start doing this.
I am signing up on the vex forum and will post what I discover.
Who is JPerlman?
I keep coming back to one thing. The only times we have ever had this problem were when we had a cell phone very close to the joystick key and when we were at our hub competition and in the arena at Texas BEST. We have given a bunch of presentations at different schools since Texas BEST and have never had the problem again.
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Interesting differences of info from VEX on the Hump.   Was my source wrong?  Do I just remember it wrong?  Oral tradition changes over time?
  Jame Perlman is the #1 answer guy on RobotC vex forum,  ~25 years engineering experience, and knows what he talks about with experiments or oscilloscope pictures and thermal imaging camera pictures to show it.   He is almost certainly the best VEX Robotics there is, and is the most helpful.

Maybe at next school presentation, you can get a bunch of students to stand close to the driver with their cell phones uploading live video by wifi, or the like,  to try to recreate the situation.      Labeling all the keys and tracking them would be a great idea.
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From Mr Pearman.
I'm not sure I'm going to be able to give much advise beyond what VEX may have already suggested. When my own teams have had similar issues we usually swap out VEXnet keys, connector problems can often be a weak point with the system, but if you have not had problems on your practice field then that's unlikely to be the issue. Everything points to an issue with radio interference. Usually different radio channels are used when actually competing, but I'm not sure if BEST is using the same field control system that we run at VEX competitions or not, I know in the past they didn't and Joel had some specially modified competition switches to mitigate this. All sorts of different RF sources can theoretically affect VEXnet. Wifi hot spots, microwave ovens, wireless phones (old school, wireless landline phones) bluetooth. So an environmental issue is most likely but always hard to reproduce and prove that was the cause.
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