[Jeff Rodriguez]

Your problem sounds very similar to ours.
We had been having alot of static build up on our robot. We multiple measures to fix it and it seemed to work. Then the same thing happened to us. We put the robot on the field and nothing. We replaced the modems, cables, everything. Tethering the robot worked fine. Eventually, we replaced the RC (missing 3 matches, oh well) and everything worked fine from then on.
The conclusion was that our static build up had fried the radio connection.

Throughout the whole troubleshooting process the IFI rep, RIcky, was very helpful.