Still not there
Pardon me for jumping in…..
I've found that the "spurts" you mention happen to me as well. Many times
it is just what you think it is. Error recovery retries. Another
possibility is that the MNP modems (and error correcting modems in general)
do it by using something akin to a file transfer protocol. The data is sent
in packets. If the packets were displayed as they were received — you'd
see garbage. So, packets are not displayed until they are fully received.
It's normally almost invisible to you unless there are errors, then you may
see pauses.