Any more input from anyone?
Steering sensor seems fine and no code stored in there; steering fluid is good; P/S pump seems functional; P/S ECU and relay have been replaced (also tested with the relay bypassed). The P/S steering always works for 60 seconds and nothing more. I have noticed a few times that after going over a speed bump, the P/S would seem to work again for a few seconds and then the steering becomes quite 'heavy' again.
I am wondering if maybe a previous owner, at some point, put some kind of stand-alone or piggy-back ECU and would have messed up the P/S somehow, not sure if that makes any sense though? I really can't think of anything else.
 
If I can't find a way to fix it, maybe I'll just remove the parts from the car to save some weight and sell them, since they are not doing any good in there. In the meantime though, any help appreciated :)