I wake many times during night with severe sweats, discomfort in chest&throat area&headache, on returning to sleep I suffer nightmares.

Nightmares. As long as you have no cardiac history then this is due to the Epinephrine excess in relation to the nightmare. Its a typical "fright-flight" response to the aspects of the nightmare. Clearly something is bothering you either in your day to day life or past experiences perhaps in childhood. A psychologist may be able to help you.