RF900DV transceiver problems with noisy signal

hello,
i'm working with the laipac RF900DV transceiver.
This is a full duplex data/audio transceiver. I want to use only data transmission.
http://www.laipac.com/easy_900dv_eng.htm
i use 31.25 kbps. When i send data since base to remote the transceiver works fine, but when i send data since remote to base i receive a nosy signal and i can't understand nothing. For this aplication i use a PIC18F452 microcontroller. The base module has a 5V power supply and the remote module has a 3.6V power supply.
I changes the modules with another with the same result.
Anyone knows that it can be happening?
I have sent a mail to custemer service of laipac without answer and i dont know what i have to do.
i attach the document that i have sended to laipac.

thanks
Alex Posada