Shortness of breath

My mother is 80.  She has been having shortness of breath with minimal exertion.  She can't even make it to the kitchen to get a cup of coffee without having to sit down to catch her breath.  She also has some dizziness with this.  She has had a stress test about 6 months ago and it was normal for a woman her age.  She had several chest xrays and an mri of her head.  All was normal.  This shortness of breath and dizziness comes and goes.  If you have a heart condition, would breathing problems come and go?  Sometimes she's go several weeks with no shortness of breath and very little dizzy spells and then, all of a sudden the shortness of breath comes back.  The doctors have not been able to solve this mystery.  Please help if anyone has any suggestions.  Thanks.