Air condition

I have Toyota corolla 2008, having Denso A/C compressor equipped with solenoid for loading and unloading.
Problem: some time it works normal, but some time especially in low speed or idling it did not work. Once again to clarify some time it works normal even in low speed and idling. When I start the engine and on the A/C some time it start working at the same time but some time it start working after a kilometer running at the speed of 40 to 60 Km/h. In high speed it gives this problem very rarely, normally it works normal in high speeds.
I check the refrigerant pressure it is normal, when working the cooling is very good. Some one was telling me the problem is in the solenoid, but if it the solenoid problem it should exist all the time?
What may the problem I want to know?

Thanks.