intermittent problem with staying erect?

I'm 25 years old and I have an intermittent problem with staying erect during intercourse. I say intermittent because there are times I can stay hard the whole time but this is rare. The rest of the time, I go soft in the middle of intercourse. My shortest time I've ever been able to finish is about 30 minutes, bit the average time it takes is about 45 to 60 minutes. I'm very fit so I know its not my heart, though my blood pressure is naturally lower than the ideal but the Dr. Says not in an unhealthy but good way. Is it because it takes so long for me to finish it just gets tired? Im just not very sensitive and takes a lot of stimulation. Also, I'm not sure if it matters but I have a history of lower back issues. I don't get pain in my lower back during intercourse but I know the nerves down there are all connected and wonder if a chiropractor would help. It's not mental, this i know for sure, quite literally when I start to go soft It's like I can't even feel anything with it... it just goes numb. I seem to stay hard in the mornings when I first wake up... but any try later in the day I have these issues.
Please help!