Kitchen sink backup

While Studor vents are good it sounds like the vent isn't opening up enough to vent the sink and if he's replacing with the same vent the same problem will persist.
Oatey has vents that address this problem. Check them out at, http://www.oatey.com/aav_public/why_vent.html
I would switch to the correct size of a Oatey Vent and see if that doesn't solve the problem. Regards, Tom

The plumber tried another vent and I believe it was the one you mentioned. The previous STuder vent worked for 5 yrs before it was replaced last year. It was suggested I may have to have a vent run outside and up the side of house. Why would previous vent work and then all of a sudden not work?

You ask good questions. Why wern't they answered by your plumber? I have a question of my own. Why do you have a mechanical vent on your sink instead of a VTR, (vent through roof)? Is this a older home?
Let's use some common sense here. A mechanical spring loaded vent works by vacume. The discharge coming out of the trap sets up a small vacume. This vacume pulls against a flap sealing off the vent and held in place by tension from the spring. When the vacume becomes stronger then the tension from the spring the flap pulls down opening up the vent until the vacume's gone and it closes up again.
You ask, "The previous STuder vent worked for 5 yrs before it was replaced last year. Why would the previous vent work and then all of a sudden not work?"
There's really only one thing that's changed here and that's the vacume needed to overcome the springs tension. Where did the vacume go? How do we get it back? About the only thing that would cause a loss of the vacume needed to activate the AAV, (air admittance vent) would be if the discharge wasn't draining fast enough to set up a vacume strong enough to open up the vent.
Now I don't know about your plumber but to me that indicates a partional blockage in the lateral drain, ( the horizontal drain in the wall) or the drain to the house main slowing down the flow so it doesn't produce the vacume needed to activate the vents flap.

"It was suggested I may have to have a vent run outside and up the side of house."
If the problem can not be solved any other way that may be all that's left. However, I'm from the old school of plumbers that believe that if a item ever worked right to began with it should be able to be repaired back to its original working order. Please keep me in the loop and let me know what's happening.
Good luck, Tom