HRM fault readings

I've been using the Garmin HR straps for years.  Old style, new soft ones and anything else in between.  Tried switching to a Polar strap that would mate with thhe Garmin HRM pod and ALL of them have demonstrated the behavior you describe.  I've tried wetting it down first, using electrolyte gel on the pads, and any other supposed remedy I could find online, but never really found a fix.  Best theory I heard was that it comes as a result of a static electricity build up from the rubbing of the strap on the skin and the shirt - don't know if it is true, but it is the best one I heard.  

After reading the DC Rainmaker review, I ordered a Scosche Rhythm+ strap that goes on your arm and is an optical sensor and have had great results and seen none of those sudden increases that take the HR into Z5 and after some indeterminate time, drop it back to the real reading.  You can see his review at http://www.dcrainmaker.com/2014/05/scosche-antbluetooth-optical.html and decide if this is a good option for you.  Good luck!