I apologize if this has been beat to death but theres a lot of information on this site and I'm new.
I picked up a bike earlier this week off Craigslist and love it aside for some issues with the shifting. First off, it has Shimano Sora components which I understand aren't the greatest but it's what I could afford.
While riding today I noticed that my bike was hesitant to shift down to the smallest ring up front, I did a little tweaking and have it moving through all 3 front rings relatively smoothly. However I've noticed in the upper gears on the 2nd and 3rd ring the chain seems to not be able to settle in smoothly on the cassette. It sounds like its popping back and forth, perhaps between 2 gears? It's only doing it when I'm pedaling heavy [up a hill for example], when I flip the bike and cycle through the gears it does everything very smoothly.
The guy I bought it from said it had been sitting inside a garage all winter so I was thinking maybe the shifters themselves needed to be cleaned and lubed after sitting in the cold for so long? If that's the case, I've read Formula 409 or Simple Green cleaner can be used to degrease them and then a spray on lube [not WD-40] can be used to lube it.
My only other thought is that the deraillieurs need adjusted because the chain rubs a bit in some of the gears as well.
Any other suggestions as to what it might be?