exercise tollerance getting worse

I am a 51 year old male fire fighter, over weight, diagnosed with a bicuspid aortic valve. I had an echo in March 2010 demonstrating mild to moderate aortic valve stenosis. Stress test in July 2010, able to exercise for over 9 minutes on the Bruce protocal. Stress test November 2010, able to access for only 5 minutes 30 seconds to 92% of maximum heart rate. Both stress tests showed a rate related left bundle branch block. My cardiologist feels the aortic valve stenosis should not be causing these results and recomended I build up my stamina by exercising on a treadmill. I have been exercising at least 5 days a week, 30 minutes or longer. I noticed that when my heart rate reaches 135 or more, I start to feel faint accompanied by slight difficulty in breathing, no angina. It seems my exercise tollerance is getting worse. My question:  Are there any underlying heart conditions I should be aware of?