it can be a few things,if it is fitted with a particle filter it could be this getting hot .if it is low milage it can be a build up of carbon deposits it the intake manifold or the exhaust system it depends on the kind of use the zafira gets.my advise is taken it down the motorway and labour the engine to clean out the engine if this does not fix it my only other advise is take it to a specialist.
i hope this helps,craig.