heavy steering

[Mac]

The problem is the steering goes from normal to really heavy to normal to heavy(moving from lock to lock) whilst youre idling( steering but not moving) the belt sqeals alot too.
When driving it is fine.

Will this just be down to a loose belt or could it be the pump/rack has gone??
The fluid level is fine maybe needs changing but will do for now.

Cheers for any feed back.