Carlton

I recently bought an '89 non-turbo car, and although it runs great there's a problem with the steering system.

When I'm going around a sharp corner (say a 90 degree bend) between 15 and 25 mph, the steering force suddenly drops as I turn the wheel past about half a turn (so there's suddenly less force required to turn/hold the wheel, and because it suddenly drops this usually causes the car to want to turn towards the inside of the turn)

When standing still, there's no problems at all, you can turn the wheel lock to lock without any strangeness. Variable assist also works fine, featherlight when parking and stiff at speed.
The car isn't lowered and has stock rims with 225/65 tires.
Power steering fluid is topped up and the belt is on tight.

Any ideas/experiences are very welcome!