When I did data communications programming as my job, my favourite tool was the HP 4951A protocol analyzer -- it would tell you anything about everything that was going on over a serial communications channel.

You probably don't have one of those .. you might be able to get a break-out box which at least would tell you if there is some flow control stuff going on. Alternatively, look for breakout box software on Google -- I'd pass you some links but either Google is slow or my connection is tired right now. Highly recommended.

Based on what little information is available, I expect it's going to be some type of flow control problem -- is there any way you can set up the serial port or the software talking to the port to enable, disable or automate the flow control?

--t. alex

"Mud, mud, glorious mud. Nothing quite like it for cooling the blood!"
--Michael Flanders and Donald Swann