More than likely it is the steering box. The links are unlikely to provide any resistance of that nature that would not appear obvious in a visual inspection (which I presume you have done).
I had a steering box failure a while back and the mechanic opened up the old box and it is full of pumps and cogs and all sorts, so in short...its a complicated affair that is not user servicable. The problems you have reported were similar to mine except mine were intermittent.