During download program intro the RCX, when about 60% of program downloaded RobotC raises error: "Failure '<transferData> transmission failure 'No response from Robot to message < <TransferData> >'. But RCX is responding' during downloading program/task code". But sometimes it didn't and program download successfully.
P.S. Sorry for my English

Make sure that your RCX is covered completely. Sometimes the IR transmitter could get interference that prevents it from successfully sending the program.

You can also try re-downloading firmware to the RCX.