I have a recurring problem with the third gear on my seven speed rear cassette. The bike is an old Cannondale with the original Sun Tour Sprint deralliuers and downtube mounted index shifters. All the components still work fine. I recently had a new rear wheel built with a new Shimano hub, and replaced the original rear freewheel with a Shimano cassette. I have a great local mechanic who did all the work. When I first got the bike back everything shifted great. After about 400 miles I started having a problem where the chain would not stay on the third gear, when using the small chain ring. It keeps jumping up to the fourth gear. I brought it back to my mechanic who told me the shifter assembly had loosened on the downtube. He tightened it, made some adjustments, and it worked great. But, about 500 miles later, its doing it again. It only does it on the third gear, and when I'm on the small chain ring. Otherwise, it shifts perfectly.
I'm curious to know if anybody has any ideas about what might be causing this. I've adjusted my derailleurs many times. But, it always been setting the high and low stops and the indexing. I'm miffed that I could be having an issue with just one of the middle gears.