Erik,
I just built this after receiving a NEO-M8N and have the following observation: Occasionally, the 1pps from the GPS skips and the software goes bonkers with a large error and makes a big step which is quickly corrected at the next 1pps update. I am guessing that the GPS signals are changing, like one or more satellites go out of range or others join.? I think the software needs to be modified so that when it gets a large error, it holds the previous correction factor.
Gary
W9TD
Gary W9TD