2003 Vauxhall Meriva lumpy / kangaroo start to engine on tickover

Car done 74,000 miles so not too elderly, but for several months has been erratic when starting up in that it will start but then jump along, a la kangaroo, until it gets going a bit more. Does this when engine or outside temperature hot or cold, ie. no particular circumstance. Been into garage twice now and problem still there - emissions light came on the other day. There is crack in exhaust manifold (although garage didn't think this was overall cause), but so far has had new lambda sensor, a broken exhaust stud drilled out of manifold, new manifold gasket fitted, substituted air flow meter but problem remained. Engine still lumpy on tickover, no fault codes on diagnostic test. Spent a few hundred pounds so far trying to solve problem and wonder if it is worth throwing more money at it or not?!! Any idea as to what problem may be? Many thanks.