2000 Dodge Neon Power steering problem

Hello there I recently had a problem with my power steering pump first in 8 years of owning the car, when the really cold weather come in(Illinois) the power steering pump started to mess up. So I had it changed, it worked for about 2 months (with loud noise and I hated driving the car, but it worked with out any problems other than the noise) so after two months of driving it went out again, I had it changed again, and same thing is happening now, I found out that in the reservoir there is foam forming when I drive or move the wheel right to left. Once I open the cap for the reservoir the air goes away and there is nothing but fluid, then it works good fro about 3 min or so, depends how many times I turn the wheel right or left. And I don't know what is causing it to foam / and I don't know how to make it stop. Thanks! Ps. My mechanic told me to wait it out and the air will go away after couple days well its been a week now and still same thing.