I haven't read aout chain/cog wear skip yet. Could your cogs or chain be worn enough to cause skipping under pressure? This is a very common problem. Every thing you said suggeste this would be the first thing I'd look for. No skip under no pressure. Only happens in some cogs (are these the most used ones)? Shifting is well adjusted and the chain remains on the same cog after the skip.

Have you taken the bike back to the shop? Some shops feel that if they do the work requested they have done a good job. Some shops don't do much of a test ride after the service. Some situations won't allow the test ride (a partially asembled bike).

Just reviewed your posts and you mention a new chain. If the cogs are worn then the new chain won't mesh with the old cogs under high pressure. The chain will climb over the teeth and the chain will skip forwards one link.

The solution, if this is the case, is to replace the cassette. Andy.