MD80 First Officer reports an altitude deviation he believed to have been caused by his inadvertent change of the altitude alert setting when he tuned the VOR.

Narrative

On arrival into the New York area on the KORRY 3 arrival; we were inside KORRY; we were given a 120 degree heading; and slowed to 210 KTS for spacing. Then the Controller gave us a descent from 10;000 FT to 8;000 FT for traffic. I was the Pilot Monitoring; so I read back; 'Roger; descend and maintain 8;000 FT for traffic.' I followed the company procedure; reset the altitude alerter; saw 8;000 FT annunciated in the lights; put my left pointer finger on the screw next to the altitude alerter and announced; '8;000 FT; set and armed' and the Captain replied; 'Roger; 8000 FT' to me. I am 100% positive that 8;000 FT was set in the window. I can still see it in my mind's eye. As the Captain began our descent; I continued getting the airplane ready for our arrival by tuning the radios in #2 for operations and setting the LGA VOR frequency on my side for the anticipated as usual; direct LGA we usually receive. It is my firm belief that when I tuned in the LGA VOR with my left hand; the pinky finger made positive contact with the altitude alerter knob and rolled the altitude down 1;000 FT to 7;000 FT. The next day; I flew the same trip; and while on the ground; I was able to do just as I described above with very little pressure from my pinky finger on the knob. By rotating the VOR knob clockwise; my pinky finger slides under the altitude knob and rolls it counterclockwise; which clicks down the altitude 1;000 FT with what seems like less than a few millimeters of subtle movement. I repeated this several times and decided it was a significant discovery. I have 7;500 hours on this airplane and can recall several times where the altitude was somehow changed without explanation. The difference was I caught it those times. The Captain was flying the airplane and began the descent. At some point; the altitude was changed. Going through 7;500 FT; the Controller said; 'Aircraft X; return to 8;000 FT.' And I said; 'Roger; leaving 7;300 FT climbing to 8;000 FT.' I did not remember being cleared to 7;000 FT; and had a bad feeling. The Controller did not say anything at the time. We did not have an RA or TA. I did not see another aircraft on TCAS; because I looked there immediately because I remembered the clearance to 8;000 FT was for traffic. When I called TRACON per their request; I told the Controller I felt bad about what happened; and that I had a theory about the altitude alert knob. After this; I realize the altitude knob needs to be more difficult to turn; or relocated; or something. I too am at fault. I should have never let the Captain descend the airplane below 7;000 FT. The alerter issue is no excuse. I needed to carefully monitor the Captain's descent and I failed to do so. I let myself get busy doing other things; depending on automation; and I know better. Perhaps fatigue is a contributing factor. I wanted a cup of coffee 30 minutes prior to the arrival; but never drink coffee this late because I can't get to sleep; which makes me tired for the trip the next night. It was the end of a long day in the seat of an MD80; schedule for 7 hours 45 minutes in the seat. Since this is something I have seen in the past and prevented it from happening; I think fatigue is a factor. I thought about this for hours after it happened; and spoke to several other MD80 pilots about the altitude alerter; and they all told me that they have experienced similar 'uncommanded altitude alerter changes' in their flight careers. Maybe now we have a clue as to how it happened.