If Viagra doesn't help, see an OPC. You have a malfunctioning spoiler by the sounds of it. I'd try and get someone to have a 'quick look' as often it is just a screw or something that can come loose.
On a slightly related topic, anyone know if it is possible to have the speed at which the spoiler raises changed by the OPC people and their laptops? I do like it popping up, but would like it to be at say 55mph/