corsa wont idle correctly

[chazcfc1]

hi,
just wondering if any1 can help me ive got a 1.6 corsa sport that wont idle correctly. when coming 2 a junction it wants to cut and and sometimes does and when it doesnt cut out its idleing to low also when it idles the revs keep bouncing about. The engine managment light is on but i have got a mantzel inlet manifold on it and have been told thats why the light is on. have changed the idle control valve before but only with a second hand 1 from my previous 1.6 engine and does the exact same 1st couple of days will b ok but then will start cutting out then have a look at it and usually its clogged up! also changed crankshaft sensor and no luck.

Any Ideas??