Thank you for your continued effort to assist me with this. I took the problem unit on a long road trip yesterday (3.6 hour drive each way) and it worked very well. It is sitting atop the Pi Zero of course. The GPS grabbed a fix and seems to have held it pretty well. I need to get the data into the database to do a proper analysis as far as changes in fix status but eye balling the logged data it looks pretty good.
I am using the assisted GPS feature. Coded that into the init routine for the GSM service as one of my first efforts to resolve the issue. It makes no difference with respect to the issues with Pi3 boards.
I will try the U-Center you reference to see if I can get some more data for us to evaluate on the issue. I will also try turning off the GSM module as suggested. Of course that would not work as a long term solution as I am using both in my product. But if it helps us figure out what is going on with the unit I am all in.
I order one of the 3 units from Amazon. It is actually cheaper on Amazon; must be the current exchange rate driving it down. I would order more but I am still in development phase and need to get a solid proof of concept out before I dump to much $ into it.
I will post more info as I get through the research on these items.