Another Serial Communication Question

I posted a question a little while back regarding a serial communication
problem I was having with a program I had ported from windows. I found that
my program, which was communicating with a device over the serial port,
generally worked, but on very rare occasions, sometimes after it had been
running and communicating successfully over the serial port for as much as
an hour, it would write a few bytes of data to the serial and then fail to
receive feedback back from the device for several seconds. The reason this
was happening was that occasionally data sent to the serial would remain in
the serial’s output buffer for about 12 seconds before actually being sent
over the serial line to the device. I originally had this problem on QNX
6.1, but now that we have upgraded to QNX 6.2, the problem is still
occurring. I have noticed, using pidin, that the serial driver (ser8250),
which usually has a priority of 10, occasionally has its priority brought up
to 24. This appears to happen when the driver delays in sending data from
the output buffer, as well as at other times. Can anyone tell me if it is
possible that the serial driver’s heightened priority level is causing my
problem, and whether there is a way of ensuring that the serial driver’s
priority level remains at 10?
Thanks a lot,