What causes periodic running induced tachycardia?

I've been running for 26 years.  I'm 57 year old male in outstanding physical health.  Over the course of the last twelve years I've had running induced tachycardia occur from once in a month to once in a week to now 3 times a week on average.  Usually I'll be running with my heart rate in the 110 - 125 range and all of a sudden my heart rate spikes up to 170 - 210 beats.  It sometimes occurs after I run a mile or two and rest and while resting jump into tachycardia.  It will  happen sometimes at the very begining of a run, in the middle of a run, resting or the end of a run.  I noticed it immediately as my heart rate shoots up.  The effort of running fast, meduim or easy seems not to matter as it occurs during all these speeds. I've tried all the remedies to try to get it back to normal but it goes down to only 105-115 level when I rest.  As soon as I start running again it gets up to 170+.  I've had an echo cardiogram with results normal. My resting heart rate in the morning is 35-39.  During the day it is in the low to mid 40's. I've never experienced tachycardia other than running.  
I run 40 miles a week.  Doctors are perplexed as to the cause.  Is it an electrical defect or trigger?

[CCF HeartMD6]

The symptoms you describe certainly are interesting -- and it sounds as if you have already had somewhat of an evaluation (i.e.. echocardiogram). One additional test, if not done already, would be a Holter monitor which would monitor not only your rate, but would allow for evaluation what rhythm you are in (i.e. sinus tachycardia, atrial flutter, atrial tachycardia, etc). Based on your reporting your AM resting HR and your rates during the day, this may have already been done. Running can induce a fair amount of catecholemines which can serve as a trigger for an even faster heart rate -- although it would be rare for it to jump so significantly, and so sporadically. Another possibility is an AVNRT.  Another important issue to address is whether or not you are having any symptoms when this occurs. If so, then I would make sure this is evaluated fully to make sure that treatment strategies are being addressed (rather than observing it).