Another remote possibility is the effect of spine problems on heart rate and blood pressure. I just had an ortho review for a full spinal exam and it's not good. My top two cervical vertebrae are in bad shape as are the lower lumbar -- mostly due to various accident injuries. Doc said that's one possible reason for my erratic HR and BP, particularly on a road bike due to the neck position -- although I often find my more upright hybrid to be more uncomfortable to my neck after a long ride.
So I'm set up for a few sessions with a chiropractor to see if it helps. I'm pretty skeptical about bonecrackers but it's what my insurance will cover, so I'll try it. I'll need to pay out of pocket for a massage ther@pist, which will probably do more good. ('scuse the odd spelling with the "@" symbol, but BF's word filters make hash of some perfectly normal words).