The MAX reliable data-rate of the receiver you have is 2400 bps. Everything
you try should be at or below 2400 bps. Over the MAX data-rate is just not
going to produce anything reliable.

Have you tried testing your RF modules with the encoder/decoder ICs you
received with your kit yet? That's what we test them with here before kits
ship.

Before moving any farther, I would definitely recommend you validate your RF
modules by testing them with the encoder/decoder ICs you received.

Once you have verified that everything's working, it's a lot easier to move on
from that point.