Hi, as norb says check the angle drive at the front of the steering box, dead easy to take off and you do not lose any fluid. If its the older type with flat roller bearings they wear then fall between the two gears giving you a tight bit.
With your symptoms the pressure relief valve on the pump could be sticking sometimes.
Hope this helps
if they really have replaced the whole system theres only the kingpins siezing up left.
but you ussually get other problems such as not self-centreing.
blipping the throttle will increase the pressure from the pump momentarily,pointing to either a worn pump,bad fluid(have a look at it is it frothy and pinky white when running?),or an over adjusted steering box(too tight in one spot-ussually to take up excessive wear).
there is a pressure relief valve in the system-this may be sticking.
dafs are prone for bursting pipes under the front grille area,and very often they are ran back to repairers which causes wear to the pumps.
If the steering has recently been overhauled it might be worth checking that the shuttle valve is fitted the right way round,but it does sound like an airlock problem.
bit confused,as you asked for advice and appear to be scoffing at evryone ,people are only trying to be helpful…apart from the angle drive .check the current flow through the steering column,as poor earths have been known to knacker the steering column ,though you normally bursts hydraulic pipes as well…just passing on what generally goes wrong with the steering after 14 years with DAF
Refurbed steering box on order 
Just aim for something soft!
Angle drive at the top of the steering box, really common on the CF, bet Ive changed 40 of them.
Hi, having the same problem with my DF cf75 310
2011 reg. It has a stiff steering and showing in read rear axle steering on the dash.