'How to lower or disable your 'fifo-buffer' on the com-port in question. Depending on the Windows version you are using do the following:

Windows NT 4
Goto 'Control panel', if 'ports' is avaible execute the link
Choose the port in question
Press 'Settings'
Press 'Advanced'
Unmark 'Fifo enable'


Windows 95-98 (all versions)
Go to 'Control Panel', execute 'System'
Find the port under 'Seriel ports'
Press 'Properties'
Press 'Advanced'
Lower the transmit and receive buffers to 1 or disable fifo


Windows 2000
Go to 'Control Panel', execute 'System'
Click the 'Hardware' tab
Click 'Device Manager...'
Click 'Ports (COM & LPT)' Click the com-port in question, e.g. 'Communications Port (COM1)'
Click the tab 'Port Settings'
Click 'Advanced'
Set both the Receive Buffer and Transmit Buffer sliders to 'Low'

Windows XP
Go to 'Control Panel', execute 'System'
Click the 'Hardware' tab
Click 'Device Manager...'
Click 'Ports (COM & LPT)' Click the com-port in question, e.g. 'Communications Port (COM1)'
Click the tab 'Port Settings'
Click 'Advanced'
Uncheck the box next to Use FIFO buffers

Windows Vista and Windows 7
Go to 'Control Panel'

Click on 'Hardware and Sound'
Click 'Device Manager...'
Click 'Ports (COM & LPT)' Click the com-port in question, e.g. 'Communications Port (COM1)'
Click the tab 'Port Settings'
Click 'Advanced'
Set both the Receive Buffer and Transmit Buffer sliders to 'Low'

If this does not help, your cable might be too long, or it might be placed next to a power cable, giving a lot of noise.