I have GI and cardiac symptoms (but no heart problems, per say). I have a history of heart palpitations. I've done the Holter monitor and had two cardiac ultrasounds. They've mostly gone away--very strange though because I've never suffered from anxienty, depression, etc. When my so-called CFS started a year ago, I actually presented to the hospital with a really low heart rate, and this has continued every time I have a flare up. I get a lot of 'heavy' feelings in my chest and I also have lots of weakness and occasional pain in my arm, especially my left arm.
And GI symptoms? Yes, indeed. I have bouts of intermittent diarrhea, nausea, and inappetance. When I have a flare up I have diarrhea daily and am unable to eat. My last flare up was in February/March and I lost 10 pounds. I've had an upper GI and colonoscopy and everything was normal. I did have an old peptic ulcer, but it was healed at the time of the endoscopy.
I've been tested for Lyme's but it was negative...
Good luck. Hope that helps.