Weird swr problem.

First off, I now know using the radios built in swr meter is not a reliable indicator. Having said that, I originally had my bone-stock Cobra 29 LX hooked up through the stock antenna/coax of my '13 Prostar and got antenna warnings. Replaced the stock antenna with a Francis 4ft. Hotrod and immediately got great swr readings (1.1 across the band on the internal meter) and decent reports of 2 miles or so on the road for the last couple months.
Today I put on a Sirio P5000 and had marginal swr readings of 2.5 or so but no antenna warning. So I took it off and put the Francis back on and now I'm getting high swrs on that, 2.1 to 2.5 and I can't get a radio check reply.
So what's changed?

 

I keep hearing that, but my truck does have a separate antenna on the right side just in front of the windshield that looks to be a normal radio antenna. What's that for?

 

Guess I'll go with new coax then. Just weird that everything seemed fine until I swapped antennas.

 

Good point. I'll check again when I'm in an open area.

 

I will have the ground checked but if I'm going to be pushing 250w out with the 98, should I even consider using the stock coax which may or may not be tied in with the tv/radio antenna? It's a company truck so I'd love to be able to use the internal wiring but I'll run and tuck new wire if need be.