Watchers question

I think I read this book 25 years ago, but I don't think it was new when I read it. Things I remember:
1. There was an older man (not sure what "older" is I was a teenager when I read it). But during the course of the book he started feeling younger. This was because he was somehow able to suck in other people's life force. He wasn't killing them.
2. He was able to see peoples life force in the form of balloon type things attached to them. This applied to animals and flowers.
3. He met a woman during the course of the book who was able to do the same thing.
4. There was some sort of goblin-reaper creature running around with scissors cutting life forces.
5. When the man and woman would suck the life force from people, the person would hear noises. I remember a waitress in a diner yelling for the line cooks to stop banging pots and pans.
6. I thought this might be a Dean Koontz book. Specifically I thought it was part of a trilogy that had Watchers in it, but I have searched high and low and I don't think it actually is his book.
It's not a lot to go on, but I think about this story every time I see cut flowers. Does anyone have any ideas?