I had this problem recently with i bike i bought through ebay, which was received in the mail. It was the 2nd time in my life I had an autoshifting problem. Both times, the culprit was a Campagnolo Downtube Shift Lever that had been lubed.
Don't lube shift levers.
Clean them thoroughly with de-greasers (such as WD-40) and dry them.
It's called "friction shifting" for a reason.
Lube everything else on your bike EXCEPT for the shifters.
- Don Gillies
San Diego, CA, USA