It's a (sewer) gas!

After recently installing a new vanity, a converted antique dresser no less, I have noticed an unmistakable odour of rotten eggs from the sink. I'm assuming this is sewer gas. The smell is only evident when the tap is running. I installed the p trap in the normal fashion, however to make everything fit the dimensions of the dresser/vanity I had to use a flexible rubber connector near the wall. This section is not absolutey horizontil, tilting downward a few degrees after the p trap. Also, if the sink is stoppered and I run the water there is no 'sulphuric' odour. It seems to me that the smell is more noticeable initially upon turning on the water, after running for a few moments it is less evident. Any suggestions? (the plumbing is PVC, the p trap has a screwed on bottom bit.)
thanks, graeme

hey Graeme,
there's a couple things that could be going on, it sounds to me like you could have too much angle from the p-trap to the drain line (the drop I think you're talking about with the rubber coupling?) which would allow the water seal in the trap to be broken by syphoning out a little bit more water than you would want to. However, you're rotten egg description is a little suspicious, are you positive you don't have sulphur in your water... you don't smell it in any other faucets in the house? While showering? I wouldn't describe sewer gas as a "rotten egg" odour, that's why I'm asking...
Does it have the same smell when you run hot and cold water separately?

odor

Pour Liquid Plumber, Drano, or similar down the sink's overflow opening and see if that cures the problem. Running water in the sink would force air through the overflow along with any odors resident in it. Stoppering the sink would not allow the air to flow and thus would not create the odor.

Thanks for the input. I checked the overflow, it is working fine (new kohler sink), but is definitely to source of the smell when the water is running. I managed to reposition the drain so there is no 'tilt' toward the wall. This may have been syphonning water from the p trap as noted by Kristi. Voila! Problem solved. Thanks guys...cheque's in the email.