Mk4 mondeo randomly stiff power steering

[lewis_04]

I bought a 2008 Mondeo estate 2.0 diesel a few weeks back and it came with a faulty power steering system, when you start the car up it’s incredibly stiff but once you’re going at a speed of around 5mph+ it feels okay again, though can sometimes randomly cut out momentarily when driving. When it’s struggling it makes a horrible hissing sound and it’s more prevalent when turning the steering wheel far left. Also for reference, when I checked the PS fluid it was pure black and lacked the consistency or smell of power steering fluid, something I’ll obviously be removing and replacing shortly hoping it’ll help or rectify the issue. 
 

I know Ford are infamous for power steering problems particularly with racks and pumps, so if anyone has experienced similar symptoms or is clued up on this subject any information or advice would be greatly appreciated as it’s just spoiling how fun the car normally is to drive.