Hi Steve. I don't think that the timout of the Serin could be of a problem since it gets reset as long as data are arriving. It times out only if there is more than the t defined idle input.

If the connection is made directly from the serial output of txer to the serial input of rxer, is the communication 100% succesful?

If it is, then I am guessing that the problem is caused by the modules. Maybe they loose sync when data that is not encoded properly are transmitted. Try to encode data like manchester or similar.

You are using a good preamble of $55, which is the best to stabilize the data slicer but your payload is not. And follows $66 that is pulling the data slicer a little.

Ioannis