I would have the doctors look into periodic paralysis.
This is a disruption of potassium metabolism and hard to diagnose. Some people don't display this until adulthood.

http://www.med.nyu.edu/patientcare/l...ChunkIID=96851

There is also a connection between low magnesium (fairly common in US with 70% of people who do not get RDAs for this mineral) and low potassium. In other words, these two electrolytes work together.

Another possibility is Myasthenia gravis which is a weakness of muscles caused by an autoimmune reaction at the neuromuscular junction. Your husband would have other signs, like difficulty swallowing, drooping eyelids, or trouble breathing in between the crisis events.

If your husband was on a bypass machine during the valve repair, here is an article that discusses the potential side effects from that use:
http://www.cincinnatichildrens.org/h...urg/bypass.htm
Have the doctors checked his kidneys? Aldosterone levels, ACE etc. The kidneys factor into potassium abberations.