Dear Mr. Zero,
I've read a description of the movie, and, despite the fact that I remember minor details a little differently, it sounds as if you're right. I believe the movie I described is, indeed, Island of Terror.
The only thing is, in one way, I find your answer to be more than a little disconcerting. You see, I have been pondering this movie, off and on, for something like 32 years. I have spent the occasional random pockets of time wondering about its title and the details, asking friends and family members about it when the conversation drifts to the subject of horrifying movies, etc. But now, having a stranger spit out a title almost as soon as I have described it leaves me, somehow, hollow and adrift without a quest.
(Just kidding.) ;-)
-Arbutus
P.S. Do you know the meaning of our existence? :-)