I'm A Fitness Trainer, Workout 5-6 Days Per Week...sometimes More

I'm a fitness trainer, workout 5-6 days per week...sometimes more than not, I experience skipped heart beats while climbing stairs or sometimes while at rest accompanied by shortness of breath.ive had multiple test done including stress test, ecogram and EKG, all came back normal.. I was diagnosed with MS and have been on Avonex for sixteen years...can these symptoms be caused by the interferon or MS itself? my neurologist doesn't seem to think so but my cardiologist thinks it's a big possibilty. Are there any test to find out and if so any medication to control this that's interfering with my lifestyle?