Vauxhall Zafira pulsing / juddering in tickover

Am now on my third starter motor and not even 45,000 on the clock ! However since my last starter motor ....car going along at 60 and flashed engine management light so lost all power and died, hairy I can tell you on a dual carriageway....the car now runs ok, but when ticking over ( first noticed 1 mile from garage at lights ) the engine seems to 'pulse' a sort of juddery tick-over. No warning lights come on. Took it back to garage and they are adamant it's not the starter motor again. They think there's an airleak or the throttle needs cleaning. Anyone experiencing same ? Garage seems clueless to me and/or fobbing me off AND IT'S AN AUTHORISED DEALERSHIP.