[Dx4win] Error messages

[Jo, YC0LOW]

When keying CW, I sometimes had consecutive two error mesages as follows:

a. Could not open COM8 for keying:ie_Open - device already open.
and then:
b. Radio time out. Radio interface has been disabled

To restore it, I must re-open the software in my notebook. I am using
ver 8.04 with Windows 7 as the OS.
I used a homemade interface made of two pcs of transistors 2N2222 and
two resistors 1.2k to my Icom 746Pro with cable USB 2.0 to serial that
has FTDI chipset.

Could someone help me to explain what would be the cause, please?
Tnx es 73 de Jo, YC0LOW