the itching is due to the alcohol content with the minoxidil. I believe you may have an allergic reaction to the propylene glycol.
http://en.wikipedia.org/wiki/Propylene_glycol
Majority of the population can only tolerate this stuff for a small period of time. A good approach would be to apply some moisturiser onto the skin such as aloe vera gel [ ps. you may get a shiney head / scalp ].
Alternatively:-
(1) purchase some minoxidil without the above chemical
(2) spectral dnc-l [which I have yet to experience any allergic reaction].
As for once a day? I believe since your scalp is now irritated, you scalp / skin will still itch.