I'm not familiar with that particular model of Tx and Rx, but as you say, if a different system worked fine then it certainly points the finger at it. Out of interest, did the robot failsafe ok?

If I remember correctly, was your robot mostly made from HDPE? Was there any chance the Rx in it was touching some metal parts inside that may have been causing issues? Maybe the radio gear is just not transmitting a particularity strong signal and in the noise of the arena and all the other radio equipment is getting drowned out? Maybe there is a problem with your radio gear?

Not much help I'm afraid but if you decide to replace it there are lots of recommendations of proven radio sets on here. From that Mark mentioned above, to the Spektrum, Orange Rx, Flysky to name but a few.