Toprol-XL and deep breath

I was started on Toprol-XL after a triple bypass in 2000. I had experienced several episodes of rapid heartbeats. For the next 7 years I took it daily and had no side effects that I was aware of other than feeling a little weak if I missed a dose.
During this time, I was diagnosed as a diabetic and started on Lantus insulin. I had some gastric side effects from the Metformin but other than that I was doing ok. I was and am still overweight but I was also in good enough shape to walk 30 minutes everyday on my treadmill and to hold down a stressful job.
In September of 2007 I hosted a large family reunion at my house and had no physical problems. Right after that I came down with a bad sore throat and was put on Clindamycin. At the same time, my husband went to refill my meds and came back with the generic version of my Toprol-XL. Our insurance now requires us to use generics if available. Within two weeks I was having shortness of breath just walking up the steps into my house. I at first thought it was a hang over from the sore throat and went to my GP. He had me switch back to the original Toprol-XL and I felt an almost immediate relief. However, the symptoms did not go away entirely, in fact after seeming better, they came back again. I went back to my GP. He was concerned enough to want to send me to a cardiologist. Since we had just moved here a year ago I had not visited a cardiologist as yet so this doctor was new to me and my symptoms.
So far we have done a nuclear stress test, echocardiogram, EKG's, heart cath, lung scan, and numerous blood tests. Everything looks good according to the cardiologist. And yet, I cannot walk through my house without stopping to try and catch my breath, I have had to quit work since I can no longer climb the stairs to the second floor offices, I cannot sweep a floor without having to sit down.
Is it possible that a side effect from this generic drug could have damaged something permanently? Has anyone else had any kind of problem like this?