This is a long shot, but...:
2 months after I finished SRT (radiation), I started having symptoms similar to those you've mentioned. The worse was exhaustion and severe heart palpitations. I ended up admitted for tests via the ER. They thought it was a heart problem but I passed all the tests (including a stress test) with flying colors.
It could well have been a delayed reaction to the radiation.
HOWEVER, I had also started a statin due to borderline cholesterol about 7 weeks before the onset of the symptoms. I stopped that drug. A few weeks later the symptoms suddenly started to resolve and they went away completely over a 3 day period.
So, is it at all possible that hubby is on such a drug?