18 GTI Loosing Tickover

Hi clever people, Im doing so work to my 1.8 Gti Cab, about every month or if I thrash my Mk1 the tickover drops off after and I have to reajust the Meter assembly and the air mixture screw to re set it

Is this likley to be a fault with the meter assembly or have people had a similar problem all help greatly appreciated

Thanks

going out of tune? or idle screw a bit loose?