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It's possible that you could have a problem in the steering gear or a binding tie rod end or ball joint, but it sounds like you have a bad intermediate shaft which goes between the steering column and the gear. The shaft contains a U joint to allow it to pivot and when this joint binds it will cause the problem you are having. The steering will get tight about every 90 degrees of travel because of the bound joint. I would recommend removing the shaft and feeling the joints by hand and you should find that one is binding and the shaft will need to be replaced.
I don't think this is the problem and as there is absolutely no play in the steering even though I can't see this joint without stripping out the dash. And I'm not happy messing with the air-bag. With constant pressure applied to the steering wheel in any one direction, if the joint had failed there would be play in the steering when turning the other direction meaning the joint hasn't failed. Another expert on another forum has recommended to park the front wheel tight to the Kerb, and then turn into the kerb putting resistance against the wheel and seeing if there was a noise or change and if there was, the belt is worn or pump is failing. yes the pump is screaming but unsure if its definitely the pump as this is an usual thing for pump to cope with ' I'll change the belt and see what happens then but don't want to by a pump unless I have to?