I've recently had this similar story on my car - car seemed to be running better than normal, but occasionally a flat spot between 2500 and 3000 revs and after about a week the engine management light would come on - diagnosis showed a potential lambda sensor problem (top ones just after the manifold). Changed just the one to see if there was any difference, cleared the light, still ran 'too well' and management light back on after another week, etc..
In the end I changed the MAF and though the fault showed lambda - the MAF cured it. Hope this helps...
In the end I changed the MAF and though the fault showed lambda - the MAF cured it. Hope this helps...