What Causes Breathing Difficulty And Palpitations?

For some months now, I have been experiencing spells that come with no warning. Some last a few seconds and others, several hours. I am unable to breathe normally and have learned recently that if I breathe through my mouth, I can stop them. I would get weak and have to hang onto something to continue standing. I ve had heart checked, carotid (sp.)arteries checked and nothing. I also had allergies tested and have been told I have none. I have had sinus for years. They are similar to panic attacks of which I had a few many years ago, but they are not the same. If I don t get them stopped right away, I experience rapid heart beat and pulse. I go weeks without having any. I m 82, but very healthy and can work for 4 and 5 hours straight gardening with no problems. The ENT doctor told me that he thought it was aging inner ear, but when I told him about the rapid pulse and heart beat, he said that wasn t the problem. Can you tell me what causes these? And what to do to avoid them. One other thing: 6 years ago I fell and hit the back of my head and experienced some vertigo for a little while. I did the physical therapy and have never been bothered again.