MPH not working

2010 SAN 230. When the boat was delivered the mph gauge would not work at all. It has now started working very intermittently! I do have the LINK system and notice that when the GPS is working the MPH is working, but when the GPS isn't working the MPH gauge is not working. Problem is they are not working a lot more than they are! Question is, are the MPH's picked up by GPS or are they picked up through the device that tells depth and temperature? So is it a bad part or just need a stronger antenna on the GPS??

There's not a dealer within several hours before you ask why don't I just carry it in.

Thanks for your help...