AC problem when engine idleing

Last week when I was driving back to home the AC was working fantastic at freeways, but when I exited the freeway and stopped at the first light the AC turns warm. Every time the engine runs under 1000 rpm AC turns warm but over 1000 rpm it became cold.. really cold at 2500 rpm. I have checked for pressure and leaks and both are OK, I also added extra AC oil to the system but the problem persist.
Any suggestion will be appreciated. The car is a Jaguar 04 X-Type 3.0 AWD
:confused: