A few things to consider.

1. If you do not clear a variable it will retain the last value placed in it.
2. Most inexpensive or sensitive RF receivers will output random noise spikes when the transmitter is idle.

Try sending data in packets of a specific size, and using some form of synchronization byte at the start of each packet. Receive your data in a specific number of bytes. If the first byte is not the synch value or character, dump the data packet.

It's rare that you'll see a solid RF application just flinging random data at a receiver.