Two different shop mechanics have tried and failed to fix your bike so I'd suspect it's something out of the ordinary. Try this:

Check all of your cable housing ends. What you are looking for is one or two tiny wires sticking out of the ferrel. If you see that, it means that the cable housing wasn't cut straight across so it is gradually shortening itself. When that happens, the indexing drifts off. After you retune it it works fine for a period of time but drifts off again and has to be retuned. Replacing the shift cable won't fix this because the problem is in the cable housing.