Chain slipping on smallest cogs (bigger chainring, new chain)
You can play around with chain-wrap and cable adjustment all you want but this problem is 99.999% of the time caused by wear on the cog's teeth. Ask your LBS if they can sell you only the cogs you need. It may seem like the cassette isn't old enough to justify being worn out, but that's a 10 tooth cog you're talking about, a tiny piece of thin metal responsible for handling all those watts we put down! ;) Plus, considering you made the switch to a larger chainring, I get the impression that you were doing a considerable amount of riding in your highest gear, accelerating the wear it experienced.