Heart rate very high hours after workout

I am a serious road biker.  I took 5 years off and put on 80 pounds.  Over the last three years I have lost 60 pounds since returning to cycling.
Last year I purchased a HRM that I was sure was defective.  About 30-45 minutes into each ride my heart rate would drop about 40 percent instantly and stay that way for 5 to 10 minutes.  Finally I went to a cardiologist who ran a thalium stress test and pronounce my heart quite fit.  He recommended that I take salt pills during extended exercise since he believed the cause of the heart rate drop was salt loss.  This worked the heart rate plunges stopped.
Now, however, I am noticing that after long rides my heart rate will not drop.  During the week I like to get 1 to 3 rides in of about 25 miles.  About once a month I will ride 50 miles or more.  
On the extended rides I have no complaints, no weakness, no dizziness or other problems.  But I have noticed that following these long rides my heart rate will stay over 100 bpm for as many as 5 - 6 hours.  My normal resting rate is in the mid 60's.
So why wont my heart rate go down?  Is this something to be concerned with?
Any advice would be greatly appreciated.
Thanks
Puckish

[bobthetrainer]

I would go back to your cardiologist for an evaluation.  It's better to be safe, especially with matters concerning the heart.