Just a short update...

I decided to try doing some work myself, to save a bit of cash. So today I gave the bike a good clean and replaced the chain. Turns out the new chain the LBS sold me was one link shorter than the one I had. And with the new chain, the previous shifting problem instantly went away. Unfortunately, the chain now skips in the middle gears (the ones I use the most), so I'm thinking the cassette also needs replacing? It is as if the chain slips and doesn't engage properly. I'll stop by the shop tomorrow and see what they way.