GX-dev connection to PLC Q4A gets lost!

hi there.
I can not establish a stable ethernet connection from PC to a mitsubishi M70 controllr that uses PLC with Q4A cpu.
this is a new CNC machine and my first experience with Mitsubishi controller [M70 series].
I need to connect to PLC to update the program with new one sent by manufacturer, and I am using Ethernt module for connecting (as instructed by the machine manufacturer).

the situation is:

• I got the icon in taskbar telling me that there is a connection established.
• I have checked IP addresses and other TCP/IP settings as per Mitsubishi manuals.
• sometimes (1 from ten! or twenty!!) I get the message from GX that it has been connected, but very soon and before I can do anything useful, it is disconnected!
• I can not ping the IP address of the CPU successfully but I have the connection icon always there
• I have added GPPW to windows firewall exclusion list, I have tried disabling firewall and anti-virus programs
• I have checked ethernet cable and testing with another one
• GX developer version that I use is 8.43.

but, all of my efforts have not given me a clue and after days, now I am exhausted of testing anything (and the manufacturer as well)!

now I am looking forward to your advices and I appreciate any helping comment in advance,
best
babak