P38 idle problems

Hi Poeple, I have an intermittant problem with my P38 when it is idleing. The engine will idle at 700 rpm all day long but then for some reason it will rise up to 1000 rpm and has gone as high as 1500 rpm, if I put it into drive or reverse then it drops back to 700rpm. I have had the car plugged into a diagnostic machine which seemed to cure the fault for a few weeks but it has started again but as I say it is intermittant, any thoughts on this subject would be greatfully appreciated, Cheers, Ian.

Model info would be useful.

Sorry, Its a 4.6 HSE 1997, cheers,

Only thing i can suggest is remove and clean idle air control valve. They get a bit sticky at times. Or it could i suppose be a speed sensor missing signals from time to time.

cheers, i'll give it a go & let you know how I get on