If the resistance checked out that means the cable and grounding is good.  Now we move onto the antenna.  What type of load does the antenna have – meaning is it a top or bottom load?  A SWR (Standing Wave Ratio) reading is basically how efficient your antenna system is.  If you get a bounce back (like a echo) off of a metal surface such as a tail gate it will trash your SWR.  I believe your firestick is a top loaded antenna (coil of wire at the top) which is a good choice.  This style of antenna is typically mounted on the body or bumper and works well as long as the load is above the roof line.  You mentioned your running a spring.  A spring is nothing more than a coil and it can radiate also – for shits and giggles try to remove the spring and see if your SWR improves.

While we are on the subject – I don't know how well you know your SWR (please don't get offended if I'm giving instructions for something you already know because I don't know your skill level).   How are you checking your SWR?  Here is a nice write up for a firestick antenna – http://www.firestik.com/Tech_D…..ng_SWR.htm