When making DC measurements on feed lines and antennas you should not expect to see nominal impedances. That 50 ohms is how RF behaves in a reactive circuit. It will not matter if your radio is on or not. Depending on how the output circuit is configured you could read anything from a dead short to high resistance. A coil of wire at RF is a reactive component. At DC it's just a wire. A basic antenna without a DC ground will always measure as open, not 50 ohms or whatever the nominal impedance should be.
The first thing that comes to mind is wind and water. SWR changing with wind should be so small you could barely see the needle move. If you're seeing it shift around I suspect something is loose, shorting, opening, or wet. A 5/8 wave CB antenna is 24 feet long. That puts a lot of stress on the mount in the wind, so much that it can disrupt the quality of the ground connection. Could the whole thing be shifting around a bit? I would start there. Then I would disconnect the cable at both ends. You should measure an open circuit from the center conductor to shield at either end. If not you've found a potential problem. It's potential because a short or resistance to DC may not be of concern at CB frequencies. However, if it's not open it's generally an indication there is a problem with your cable or a connector. Then create a short at one end. You should measure nearly a dead short at the other. There will be some small resistance over the length of the cable and back, a few ohms.
I'm not familiar with this antenna's construction but the features list says DC ground. So measuring it will likely show a DC short and tell you nothing about its tuning or function. This can make such troubleshooting difficult.
The features list says it offers SWR tuning, well, tune it. If you're getting 1.5:1 over most of the band I would suggest it's okay but could be better. Is it higher at one end of the band than the other? That's a sign of needing to be tuned.
I love marketing like this, this thing says it handles 5000 Watts. HA!, for 2 milliseconds maybe.
A coil of several turns of coax can help prevent lightning damage by imposing a very high impedance to lightning making the cable an unattractive conductor. It may be very effective in keeping small static discharges out of the radio. It is wise to install a grounding block at the point of this coil to insure there is an alternate path for the current to take. Electricity, if pushed hard enough, that is with enough voltage, will go through absolutely anything, wood, glass, ceramic, fiberglass, you, anything. It might destroy the material on the way but it's going through.
