Strange Falcon EF Air con problem.

Hi.

I have a strange problem with my aircon on my Ford Falcon Futura EF 1995.
When the engine is cool the aircon starts without any problem.
But when i start with a warm engine the aircon compressor do not engage.
BUT, and this is the strange part, when i rev the engine to between
2500-3500rpm the aircon starts working again.

Anyone had this problem before or have any idea on what it could be?
Its driving me nuts. :-/


Thanks
Peter