GPS delayed refresh

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I had several scary experiences with my E Series chart plotter on a recent trip from P.R. to Florida. In tight navigational situations e.g. entering a narrow harbor with surrounding reefs or rocks (with chart plotter zoomed in to a very small area) it displayed my course wildly off from my actual course - sometimes as much as a 1/4 mile. Then suddenly it would correct itself and show my true position. I could have easily run aground or worse. Fortunately sight navigation, common sense and an iPad back up alerted me to the problem before it was too late.

Because the Navionics for iPad GPS worked properly evidently it is not a problem with the GPS system itself. I have read that the military will deliberately skew GPS at certain sensitive locations. I was in very remote areas of the Outer Bahamas.

The firmware and software had been properly updated before the trip. The problem had not manifested on other cruises in the islands.

The GPS antenna is mounted on the port stern rail. see photo attached.

My theory is that a support leg for the arch which passes a couple of inches astride of the GPS antenna and in direct line between the antenna and the chart plotter at the helm is interfering with the SeaTalk signal from antenna to chart plotter. I am contemplating moving the antenna from the stern rail to the top of the arch. Does this seem like a plausible explanation for the delayed refresh when tightly zoomed in? Any concerns about relocating the antenna to the top of the arch where it would be free of any obstructions?