1999 Ford Windstar - Air Conditioning blowing warm at idle.

I have a 1999 Ford Windstar SE minivan with a 3.8L v6 engine. The air conditioner blows cold at highway speeds but it blows cool/warm at low speeds or idle. I have just replaced the serpentine belt to see if that would help and it was time anyway and it has not. The pressure from the low pressure valve is in the normal range, so the refigerant seems fine. Any help would be appreciated.

Thanks,
Chris Messer