1992 mercedes 190E stalls when coming to stop and changing into second?

Car stalls when coming up to a roundabout if i come down through the gears. if i go from 4th to neutral it doesn't always stall. If i accelarate hard then slow down it stalls. Removing the gas / petrol cap seems to fix the problem. There is a lot of suction whoosh when i undo the gas cap. My brother thinks i may have the wrong gas cap and need a ventilated cap not the one i have. Has anyone heard of this before?