Dear Bill,
Weakness is one of those symptoms that can sometimes be hard to pinpoint the source. It could be part of your recovery or due to your medications as your doctors have investigated. It could also be due to another problem such as a viral infection. I would keep working with your doctors to find a cause. It is also possible this will clear up on its own and not need any additional work-up.
To Bill,i had triple by -pass last year, i too was getting weak spells, are you on a beta -blocker?, i was and this was the culprit, also lipitor can cause this. Relax, try not to center your attention on your feelings concerning your heart to much, and also, you may be pushing yourself to much, to soon.
Good luck,
Paul