Rich,
random errors could be due to interference with other devices in the neighbourhood. (e.g. weather stations etc)
The ISM band (see http://en.wikipedia.org/wiki/ISM_band ) is pretty wild west. The only way succesfull transmission is possible is because it is assumed (dangerous word here 
) that all transmitters only send a small percentage of the time (e.g. less than 1%). Most devices I have seen will send the same message 3 times, hoping that at least one instance will survive collisions with other transmissions. As 915Mhz is uncommon in Europe, I get perfect transfers 
, as soon as I get 868 working I expect to see more collisions.
Alternatively one could use a ack protocol to get a more robust stack. Or if you want to be creative (:-)): something like automated channel hopping, or a micro tcp/ip stack
Cheers,
Hans