Power steering failed?

p steering failed on my 04 vauxhall astra 1.6,iv had a new pump fitted,my alternator checked ,and my fluid checked.the steering seems ok first thing in the morn but 5 mins down the road i can fel it kick in and the steering is really hard to drive ,sometimes if im turning it actually loosens for a few seconds then goes back hard straight away,any budding mechanics know what could be wrong with it,many thanks