For one thing, I would never try to transmit more then half the maximum baud rate of the specified Tx/Rx pair.

Also, a manchester encoding/decoding scheme is a must for such RF links.

More to this, check if the power supply to the Receiver is very clean. If necessary add extra R's and C's to filter more the power from your sepparate 78xx chip. Large capacitors are good thing to consider.

Ioannis