welcome to healthcare magic forum
you need not to worry about it
it may be due to stress
be relax ,be positive
main center for the erection is in mind ,so more you fresh and free ,more you enjoy sex .
as far as erection is concern it is not possible to have such a rhythm of erection .
do regular exercise and yoga
eat healthy diet .
be frank with your partner ,feel free to talk with her in any subject
i hope i answered your question