MorbidMeathead

So when I first start the car it will rev around 1000 rpm then once warmed it will drop to about 600 then keep going back n forth. And when im down in the 800-600 range I start getting a slight knock in my break pedal...
I keep thinking the knock is maybe a motor mount since she's hitting low revs and almost stalling out.
Pretty sure the pcv got changed out 20,000km ago
any thoughts???