1999 Vauxhall Astra manifold

there could be a few reasons as to why this is happening. i would check the plugs are correct. its possible that you may have the incorrect heat range and the chambers are getting too hot. Is any coolant or fluid dripping on the manifold? or any worn or broken engine mounts? check all those kinda things to try to explain the manifold. hope this helps!!

there is a lot of faulty manifolds being sold at the moment not being heat treated properly so all i can say is take the car back to who ever you had do the work who can varify that the car has had a new manifold via the dealers and you should have parts warranty any way so you have nothing to lose

Manifolds on these are very common for cracking, Urs should be under warranty anyway.