I have a 53 focus st170. Had similar problem with really bad lumpy idle on tickover after engine warm up & a slight juddering around 2000rpm. Diagnostic check, replace before cat lambada sensor. Still had problem. Replaced spark plugs, ht leads & coil pack, still had problem. Cleaned throttle body & butterfly, cleaned air box breather pipe, cleaned maf sensor & replaced air filter. Still had problem. Went through pages & pages of posted threads & came across a topic on crankcase breather pipe situated behind throttle body & other end going to engine block under coil pack. Sure enough this pipe had a small split in it common problem after time. Gk ford £15. Replaced pipe & runs like a dream now. So after going long way round as some of us do from time to time & emptying pockets of cash. A quick vacuum check would have done job. Where would we be without forum sites. Thanks