Help me solve a problem in an interface for COM1 and AT89C2051 communication

Hello,

I built an interface that connects the computer serialport (COM1) to
an AT89C2051 Microcontroller using an FM transmitter and Reciever.
(Telecontrolli RR3 and RT4 at 433Mhz)

I set the link to be used with the following paramaters:
no parity, one stop bit, 1200 baud rate, 8 data bits.

From the computer I run the line through a MAX232 chip to change its levels.
Each time I send data I can see that there is a delay of about 30ms until the next byte is sent. (of course that when the line is idle, it is held at 'high')

The problem is, that the reciever, after 25ms or so of recieving a continious 'high' level lowers it's output to 'low', until recieving '0' again which then will make it output 'high' if it is sent from the transmitter.

this makes communication impossible, because the microcontroller, doesn't see the right Start bit (which is also '0').

Does anyone have any tip for me on how this can be fixed?
I used C# to program the controller software.

Thanks,
Or