Can you get a raging hard on with adult entertainment if the GF is not around? The thing about erections is if you do not initially get fully erect, you have venous leakage because there is not enough pressure on the tunica to keep the blood in the penis. If you are thinking about anything non sexual when erect, down it goes. Any anxiety or loss of concentration when having sex will cause you to loose it. I have a problem always thinking about work, trust me in the heat of passion all it takes is one thought of work and down it goes. You might just need to pay more attention to the task at hand, No pun intended...
If you go to the doctor for tests, ask for Glucose, TSH, Total Testosterone, Bioavailable Testosterone, LH, FSH, Calcium, ACTH and Cortisol