You are 76 years old, have sex once in 2-3 months, what more do you want? I think you are getting greedy. People who are much younger cannot boast of having sex every day, week or even every month.
As you age, the levels of testosterone decrease and this causes less flow of blood to the penis, which results in semi erection. Can you run at your age? No, why? Because the body is ageing and this is normal.
Do not compare yourself with teenagers or men who are in their thirties and forties. Be wise. Use your body intelligently. You do not need a rock hard erection at this age to please your partner. What is needed is not penetration, but lots of caring and hugging and close body contact. Your wife would also not appreciate vigorous penetration at this age.
There are some reasons which may not be age related. Have you checked for Diabetes, Blood Pressure, Heart diseases, enlarged prostate etc. Diabetes can damage the nerves that signal increased blood flow to the genital area. A man with type 2 diabetes is twice as likely to have low testosterone compared to a man that does not have diabetes. Your doctor can test for diabetic nerve damage and low testosterone. Also, any constriction of blood flow from heart disease and artery blockages would hamper an erection.
Do you consume alcohol, or take any medication? Alcohol, tobacco, and certain medications can also cause ED. Alcohol slows nerve communications within the brain, which can affect arousal signals and physical coordination. Tobacco not only restricts blood flow, but can lead to serious diseases that may further impair sexual function.
Medicines can also affect people differently. A drug that decreases sexual performance in one person might not in another.
Don’t get stressed. Your participation on this Site shows that you are sexually aroused and that is sufficient.