I have never used those but I have used Linx Technologies LR series modules. I found that without a 10K pullup resistor (to +5V) on the RX side, they would not work at all. The TX side did not need anything. Are you making only a transmitter and receiver or are you making two transceivers. Don't forget that you cannot have to transmitters transmitting at the same time. With the Linx modules, if you don't pull down the PDN line, the transmit port will always be open making it impossible to send data. Of course, if only using one TX and one RX, this is not a problem. The other thing you may look at is interference in the surrounding area. Let me know if you still have problems.