BMW 320i Exhaust fumes inside cabin

I've bought this used BMW 320i 2006 recently (37k mileage), I get intermittent exhaust fumes through heater inside the cabin. This usually takes place when I slow down, though not all the times. Sometimes I feel the car acts sluggish and lazy, not sure if these two are inter-related. I've shown it two three garages: G1 said no problem detected at all, G2 said no leakage on the exhaust was found, but advised that the HC emission in low revs are high (248 ppmVol) and said catalyctic convertor is 'lazy' and this is the cause of the problem. Finally, took it to BMW main dealership, and after charging me £100, they said the problem is due to vacuum pump and rocker gasket cover leaking oil, and that infiltrates through heater vent; costing me £800! They said no problem with the exhaust leakage or emissions! Absolutely puzzled with so many different diagnoses; anyone have an idea?