Im not clear, but are you leaving A7 running DURING the unplug-replug process?

Also, try testing it using non-arduino software. (Say Hyperterminal on XP and below.)
It may be re-connecting on a different com port and the arduino software prolly doesnt care.

I did all my testing with hyperterminal <-> A7, a real serial port at the hyperterminal 'end'
and a generic usb stub that run with just windows built-in drivers at the A7 end.
Sometimes swapped over to ensure it worked the same way in both directions.

Another possible issue may be if the drivers are still initiallising when
A7 is started, it may not see that serial port yet.

I'll be listening...