Forester 2004 2.0 - no power when warmed up?

Hello guys, I am having troubles with my 2003 Forester. When the engine is cold, car works perfectly, but when the engine is warmed up on working temperature, the car has no power until 2000-2200rpm, Then it pulls like turbo. I noticed that, when I come to the hill and if the rpms are under 2000 I cant drive up the hill in second gear, I need to put in a first gear, but when the engine is cold, car works perfect, I can get to the top of the same hill with less rpms than 2000.
Its really annoying... any suggestions?

Regards Blaz